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THE 

SCOVILL 

PHOTOGRAPHIC 

SERIES 



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Send for Book Catalogue. 423 Broome St., NEW YORK CITY. 


THE CHEMISTRY 

OF 

PHOTOGRAPHY 


W. JEROME HARRISON, F. G. S. 

Chief Science Demonstrator for the Birmingham School Board, 
England ; Author of the “ History of Photography,” 

“ Photography for All,” “ Ci-iemistry for 
Beginners,” Etc., Etc. 



New York : 

THE SCOVILL & ADAMS COMPANY 

1892 




I'KE GOT/ CENTER 

lIBBARY 


PREFACE. 


The object of this book is to assist in the abolition of 
^‘Photography by Pale of Thumb.” 

The study and practice of the “ light science” (as CutJibert 
Bede playfully styled it forty years ago) ought to be educa- 
tional as well as recreative. But even from the point of view 
of the man who cares nothing for photography save as a 
means of making pictures, it cannot be doubted but that the 
student who thoroughly understands his tools, and who is able 
to give “ a reason for every rule,” will turn out a far higher 
percentage of technically successful work than one to whom 
all chemicals are merely “ portions of matter” ; and to whom 
every change, every reaction is “ magic or mystery.” 

Familiarity with our modern miracles has, it is true, bred 
contempt in the minds of many ; but every true craftsman 
tries to get below the surface of things, and to understand the 
nature of his materials and the way in which they act and 
interact. 

In my account of many of the chemical processes employed 
in photography, I have treated the subject historically as well 
as scientifically. For I think that we ought to recall as often 
as possible the names of the many workers who have helped 
to place photography on its present high footing as “ the 
handmaid of all the sciences ” ; and besides, it will generally 
be found tliat the pith of a matter is most forcibly presented 
in the wmrds of the man who was the actual discoverer. 

I wish to take this opportunity of expressing the special 
debt which I owe to photography for the many kind friends 
with whom it has brought me into relation in both hemi- 
spheres ; and I wish to thank those friends for their kind 
appreciation of my labors. If this book — used either as a 
text-book or as a work of reference — should prove in any way 
useful to the daily increasing fraternity of the “lovers of 
light,” I shall esteem myself amply repaid for the time and 
pains (not inconsiderable) which I have expended upon its 

preparation. WILLIAM JEROME HARRISON. 

Birmingham, England, hlovember, 1892. 




CONTENTS. 


PAGE 

Introduction, 1 

CHAPTER I. 

Matter and Force, 3 

CHAPTER II. 

The Chemical Elements, 9 

CHAPTER HI. 

Terms Employed in Chemistry, 14 

CHAPTER IV. 

Chemical Laws and Theories, ------- 18 

CHAPTER V. 

Chemical Manipulations, -------- 22 

CHAPTER VI. 

Preparation of Gases, 33 

CHAPTER VII. 

Books, Apparatus, Chemicals, ------- 39 

CHAPTER VIII. 

Treatment of Residues, -------- 47 

CHAPTER IX. 

Table of Chemical Elements and Compounds Commonly 

Employed in Photography, ------ 55 

CHAPTER X. 

Chemicals Employed in Photography — Acetic Acid to Alde- 
hyde, ----------- 59 

CHAPTER XI. 

Chemicals (continued)— Aluminium to Bromine, - - - 66 


VI 


CONTENTS. 


CHAPTER XII. 


PAGE 


Chemicals— Cadmium to Hypochlorous Acid, 


76 


CHAPTER XIII. 


Chemicals — Iodine to Zirconia, 


107 


CHAPTER XIV. 


Chemical Composition of the Sensitive Surfaces Employed 
TO Retain the Camera-Image in Photography, and the 
Chemistry of Their Preparation, 168 

CHAPTER XV. 

The Chemical Action of Light — Nature of the Latent Image, . 181 

CHAPTER XVI. 

Theory of the Latent Image (continued), . - _ . 188 

CHAPTER XVII. 

Physical Theories of the Latent Image, . _ . - 196 

CHAPTER XVIII. 

The Chemistry of Development— (L) Daguerreotype Process, 204 

CHAPTER XIX. 

Chemistry of Developing Processes — (II.) Calotype and Wet 

Collodion, 209 

CHAPTER XX. 

The Chemistry of Alkaline Development, _ _ _ _ 214 

CHAPTER XXL 

Chemistry of Development — (HI.) Bromide of Silver in 

Gelatine, 219 


CHAPTER XXII. 

Chemistry of Alkaline Development (concluded). 


224 


CHAPTER XXHI. 


Orthochromatic Photography, 


228 


CONTENTS. 


Vll 


CHAPTER XXIV. 

PAGE 

Chemistry of Silver Printing — Matt-Surface, Albumen, Col- 
lodion AND Gelatine Prints, 247 

CHAPTER XXV. 

The Carbon Printing Process and its Chemistry, - - 258 

CHAPTER XXVI. 

Printing with Salts of Iron — The Cyanotype and the Kalli- 

TYPE Processes, 266 

CHAPTER XXVIl. 

The Platinotype Printing Process and its Chemistry, - 272 

CHAPTER XXVIII. 

Reducing Processes and their Chemistry, . . . . 282 

CHAPTER XXIX. 

Intensifying Processes and their Chemistry, - - - 305 

CHAPTER XXX. 

The Toning of Photographs Considered Chemically, His- 
torically AND Generally, 334 

CHAPTER XXXI. 

Toning of Photographs (continued), - . _ . . 350 

CHAPTER XXXII. 

The Chemistry of Photographic “ Fixing” Processes — I. Early 

Methods, . 375 

CHAPTER XXXIII. 

The Chemistry OF “Fixing” Processes (continued)— II. “ Hypo,” 

“ Cyanide ” and Water AS Fixing Agents, - - . . 339 

CHAPTER XXXIV. 

The Chemistry of Hypo Eliminators, - _ _ - . 499 



INTRODUCTION. 


There can be little doubt but that tlie photographers of 
twenty years ago were better chemists than the photographers 
of to day. Our work is made easy for us by the manufac- 
turers of pure chemicals and excellent dry plates ; but the 
labors of our predecessors were not without their recompense; 
and although we have gained much in convenience by being 
no longer under the necessity of preparing our own chemicals, 
and studying the behavior of the silver bath, yet we have also, 
to a large extent, lost the love of experiment and research 
which enabled the last generation of photographers to make 
such remarkable advances. 

No one wdll deny that a knowledge of chemistry is absolutely 
necessary to the proper comprehension of photographic pro- 
cesses. Photography is, indeed, but a branch of applied 
chemistry, so far as most of its manipulations are concerned ; 
and the photographer who is ignorant of this science is only a 
worker by rule of thumb.” But chemistry itself has advanced 
very rapidly — has been almost revolutionized, indeed — during 
the last quarter of a century. It was with pleasure, therefore, 
that I acceded — a year or two ago — to the request of the 
editor to contribute to the Photographic Times a series of 
articles upon modern chemistry, considered more especially in 
its relations to photography : these articles were followed by 
others in which photographic materials and processes were 
treated from a chemical point of view ; and the present book 
consists of these contributions, with such alterations and addi- 
tions as have been found necessary. 



The Chemistry of Photography. 


CIIAPTEE I. 

MATTER AND FORCE. 

Existence of Matter . — To all substances which are able to 
affect any of the senses we give the name of Matter. Thus 
matter is a general name for everything which helps to com- 
pose the crust of the Earth and its liquid and gaseous envelopes 
— the ocean and the air. Then there is matter external to our 
Earth. Our eyes tell us of the existence of countless heavenly 
bodies — the sun, moon, and stars — and these are connected 
together by a mysterious kind of matter called the ether, of 
which we know very little, but which tills all the space between. 

General Properties of Matter .— the fact that mat- 
ter always appeals to the senses, we recognize its existence by 
the fact that it possesses certain general properties, or proper- 
ties which are common to all kinds of matter. 

Thus all matter has loekfit. A cubic foot of air weighs 
nearly 1 J ounces ; and a cubic foot of the lightest kind of mat- 
ter known — the gas called hydrogen — weighs nearly tliirty-five 
grains. 

Pimsibility — or the capability of being divided— is another 
characteristic of matter. Some solids possess this property in 
a very marked degree ; thus gold can be beaten out so thin that 
it would take three hundred thousand leaves laid one upon 
another to make a thickness of one inch. A grain in weight 


4 


THE CHEMISTRY OF PHOTOGRAPHY. 


of the odoriferoHS solid called musk will scent a room for years^ 
with little, if any, perceptible diminution in weight. How 
extremely small the particles must have been, which, proceed- 
ing from the musk, spread through the air and so caused the 
odor ! 

But it is wBen solids are dissolved in liquids, or heated till 
they change into gases, that the most remarkable facts about 
the divisibility of matter become known. A very small lump 
of sugar will give a perceptibly sweet taste to a quart of water. 
There must be tiny pieces of sugar in each drop of the water, 
though they are so small as to be invisible. As much strych- 
]iine as can be taken up by the point of a needle will give a 
distinctly bitter taste to the whole of a pint of water; and a 
single drop of any of the aniline dyes will color a large quan- 
tity of the same liquid. 

Extension^ or Enpenetr ability ^ expresses that property of 
matter in virtue of which it takes up space, or occupies room. 
Two things cannot be in the same place at the same time. 

Porosity implies that all substances possess pores or hollow 
spaces. Solids vary in their porosity, from those like sponge 
— which is highly porous — to glass, in which the pores are very 
few and small. The porosity of liquids can be shown by dissolv- 
ing a small quantity of salt or sugar in a given bulk of water 
contained in a glass vessel. When the solid has all dissolved, 
it will be found that the liquid containing it occupies no more 
space than it did before the solid was added. We can only 
explain this by supposing that the solid becomes divided into 
extremely small parts, and that these tind room in the pores 
of the liquid. 

The porosity of gases can be jaroved by putting a few grains 
of solid iodine into a glass flask and then carefully corking it. 
When the flask is gently heated the iodine will be converted 
into a violet vapor which will fill the flask. Thus the flask 
will now be full of two gases — air, and iodine vapor — and this 
can only be by the latter finding room in the pores of the 
former. 

Special Properties of Matter . — But while all matter has 
certain properties — such as weight, extension, etc. — incommon^ 


MATTER AND FORCE. 


5 


each kind of matter is distinguished by possessing certain pro- 
perties, which the other kinds do not. Thus the color of matter 
varies greatly ; and in hardness, solubility, and many other 
points there is gi’eat diversity. It is the province of the 
chemist to make himself acquainted with these similarities and 
dissimilarities of matter, for he is thereby enabled to recognize 
the different kinds of matter, and to distinguish them one from 
another. 

The Three States of Matter. — Matter can exist in three 
states — the solid, the liquid, and the gaseous. Moreover it is 
possible by the addition or subtraction of heat, to change the 
state of almost every substance. Thus, taking water in the 
liquid state, we can by raising its temperature to 212 deg. F., 
convert it into water-gas (or steam) ; while by withdrawing 
heat until the temperature is reduced to 32 deg. we can cause 
it to assume the solid state known as ice. 

Matter is indestructible. It is impossible to destroy or 
entirely get rid of any portion of matter whatever. We may 
break, burn, or dissolve a substance, or change its appearance 
in many ways, but we cannot destroy it. We can prove this 
by showing that the weight of the original sulistance is always 
present. Thus when a candle burns, the matter forming the 
tallow of which it is composed is not destroyed. Tallow is 
mainly composed of the two elements hydrogen and carbon. 
The hydrogen and the carbon each unite with the oxygen 
of the air to form, respectively, water- vapor and carbonic 
acid gas. These gaseous products mingle with the air and 
usually escape unseen, while the candle disappears. But 
if, by chemical means, we arrest the escaping products, we 
shall find that their weight will exceed that of the candle, for 
they will include all the matter which formed the candle, and 
in addition some oxygen gas from the air. Thus we do not 
destroy the matter composing a candle by the act of burning 
it, we merely change it into other substances — water- vapor and 
carbonic acid gas. 

The Structure of Matter. — The discoveries of modern science 
have led us to believe that matter consists of exceedingly small 
parts called molecules. When we take a lump of sugar and 


6 


THE CHEMISTRY OF PHOTOGRAPHY. 


grind it in a mortar we obtain numerons particles of sugar. 
Although these particles are small, they are perfectly visible 
to the eye. But shake up some of these sugar particles in 
water and they become invisible ; the sugar dissolving^ as we 
say, in the water. It is in the water, but it is now in pieces 
so small — called molecides — that they cannot be distinguished 
even by the aid of the most powerful microscope. Thus we 
regard every portion of matter, whether solid, liquid, or gas- 
eous, as composed of an enormous number of small parts or 
molecules. 

Although we cannot see these molecules, yet mathematicians 
and physicists have attempted to estimate their size, and Sir 
William Thomson states that “if a drop of water could be 
magnified until it appeared the size of the Earth, then the 
molecules of which it is composed would be visible, and would 
be of a magnitude somewhere between that of a grain of sand 
and cricket balls.” 

It is needless to say that we can never hope to attain such 
a power of magnification, and therefore we can never hope to 
see the molecules. Yet, thanks to the researches of the chemist 
and the physicist, we are as sure of their existence, and can 
reason upon their powers and properties with as much cer- 
tainty as if we could handle and see them individually. 

Molecular Motion , — No molecule forming part of any por- 
tion of any kind of matter is ever at rest. In a solid the 
motions of the molecules are necessarily much restricted by 
the force of cohesion, but in a liquid they have more freedom 
and can roll over one another; while in a gas the course of 
any molecule is only checked by its concussion with its neigh- 
bors, or by the sides of the vessel or room in wdiich the gas 
happens to be contained. 

Molecular motion is usually of a vibratory or to-and fro 
motion, like that of the prongs of a tuning-fork. But the 
vibrations of the molecules do not affect our senses in the same 
w^ay as the vibrations of the tuning-fork. The impressions 
which are produced by molecular motion are those of heat and 
light. 

When a piece of iron feels cold it is l>ecause the molecules of 


MATTER AND FORCE. 


7 


the iron are vibrating very slowly as compared with the mole- 
cules of which our skin is composed. When the same piece 
of iron feels hot, it is because the rapidity of motion of its 
molecules has in some way or other been greatly increased. 
Lastly, when the iron becomes red, or even white hot, it is due 
to the fact that the iron molecules are vibrating with almost 
inconceivable rapidity, and are then able to produce waves of 
light in the ether which lies between the iron and our eyes. 
These waves in the ether traveling through the intervening 
space, at last reach the retinal expansion at the back of the eye, 
wdiere they excite fresh vibrations in the optic nerve, and these 
traveling along that nerve to the brain, produce there the sen- 
sation of light. 

Changes 'Due to Molecular Motion . — The effect of heat upon 
a body is to throw its molecules into more rapid vibration. 
This motion causes each molecule to push away its neighbor, 
with the result that the body expands or becomes larger. 

If the heat be increased beyond a certain point, we have 
learnt that solids are changed into liquids, and liquids into 
gases. A still further increase of heat may produce a still more 
remarkable change. By heating many substances it is possible 
to convert them into two or more new substances of very dif- 
ferent appearance and properties. Thus, if a little of the red 
precipitate ” of the drysalters’ shops be strongly heated in a 
glass tube, it entirely disappears and we have in its place two 
substances known as ^‘oxygen” and mercury.’’ The exj)la- 
nation of such changes belongs to the science of chemistry. 

The Forces of Nature . — Although we cannot conceive of 
force” apart from ‘‘matter,” nor of the existence of matter 
unacted on by force, yet we believe them to be radically diffe- 
rent. Force is without weight — a body wdien hot weighs no 
more than when cold — and is unable to affect tlie senses except 
through the medium of matter. 

The forces of nature have been classified as follows : 


I. — The Physical Forces. 


Gravit}^ 

Electricity, 

Magnetism, 


Heat, 

Sound, 

Light, 


Cohesion. 


8 


THE CHEMISTEY OF PHOTOGKAPHY. 


These physical forces are able to act at a distance. To the 
action of gravity and of light, for instance, no limits can be set ; 
the effects of these forces extend far beyond even our solar 
system. 

Sound can travel by means of vibrations of the air, but not 
by the ether, and earthly sounds are, therefore, confined within 
the limits of our atmosphere. 

Cohesion is unlike the other physical forces in being unable 
to act save at comparatively small distances, but even these 
distances are wide, as compared with those over which the 
chemical force is able to exert its influence. 

The physical forces do not commonly change the properties 
or composition of the matter upon which they act. F or example? 
apiece of iron may be allowed under the influence of gravity 
to fall from any height ; it may be melted by heat / after cooling 
it can be magnetised by causing a current of electricity to flow 
round it ; yet, after all these physical forces have acted upon it, 
the iron is still unchanged in its nature and properties. 

II. The Chemical Foece. — The chemical force stands apart 
from all the other forces in two respects. It is quite unable 
to act at even extremely small distances ; indeed it seems 
necessary to bring the molecules of one body into actual con- 
tact with those of another before chemical action can take 
place between them. For this reason it is hardly ever possible 
to get two solids to unite chemically ; one or, better, both 
must be brought into the liquid or the gaseous state. 

Then the chemical force always produces a change in the 
composition, and usually in the properties of the matter upon 
which it acts. When sulphuric acid, for example, is poured 
upon sugar, violent chemical action takes place, gases are 
given off, and a black porous solid remains. 


CHAPTER IT. 


THE CHEMICAL ELEMENTS. 

Definition of Chemistry . — Chemistry is the science which 
studies the composition of matter, and which endeavors to 
explain the processes by which changes are effected in its 
composition. 

So great a mass of chemical knowledge has now been accu- 
mulated that a lifetime might be spent in the study of a single 
section of chemistry. In these articles we shall only consider 
those ])rinciples and facts of chemistry which bear more 
especially upon photography. 

The Two Great Divisions of Chemistry. 

The science of chemistry is divided into two main parts ; 
viz : — 

I. Inorganic Chemistry, dealing with substances belong- 
ing to the Mineral Kingdom, i. e.^ with matter having no parts 
or organs ; and, — 

II. Organic Chemistry, which treats of compounds many 
of which exist ready formed in animals or plants — organized 
beings. 

Organic Chemistry is also known as the chemistry of the 
Carbon Compounds, because Carbon forms a part of every 
substance of which this division of chemistry treats. 

Nature of the Chemical Eh ments . — We are acquainted with 
about seventy substances, each of wdiich we believe, in the 
present state of our knowledge, to be composed of one kind 
of matter only, and that different from all other kinds. These 
seventy substances are known as the chemical elements. Thus 
an element may be defined as a simple substance^ composed of 
one kind of matter only. 

Gold is a good example of an element. Out of pure gold 
no one has succeeded in getting anything but gold. 

It is impossible to change any one element into any other 


10 


THE CHEMISTRY OF PHOTOGRAPHY. 


element. This was the mistake made by the alchemists^ who 
in the Middle Ages tried so hard to transmute the baser metals, 
such as lead, into the precious metals gold and silver. The 
molecules of lead are totally different in their properties from 
the molecules of gold, and it is equally impossible to change 
lead into gold, or gold into lead. 

Still there is no reason why the number of the elements 
should not any day be increased or diminished. Within the 
last ten years six new elements have been found, though all 
these are of rare occurrence. On the other hand, it is just 
possible that the continued researches of chemists may end in 
proving that some of the substances we now consider elemen- 
tary, are really compounds of two or more other elements. It 
has even been suggested that there is only one true element, of 
which all the others are composed in varying proportions and 
under varying conditions ; but we are still very far from being 
able to prove this. 


TABLE OF THE CHEMICAL ELEMENTS. 


Name. 

Symbol. 

Atomic 

Weight. 

Name. 

Symbol. 

Atomic 

Weight, 

^Aluminium 

AI. 

27 

*Gold 

Au. 

196 

Antimony 

Sb. 

120 

■^Hydrogen 

H. 

1 

Arsenic 

As. 

75 

Indium 

In. 

113.4 

*Barium 

Ba. 

137 

*IODINE 

I. 

127 

Beryllium 

Be. 

9 

■^Iridium 

Ir. 

192.5 

Bismuth 

Bi. 

208.2 

*Iron 

Fe. 

56 

Boron 

B. 

11 

Lanthanum 

La. 

138.5 

*Bromine 

Br. 

80 

*Lead 

Pb. 

206.5 

*Cadmium 

Cd. 

112 

Lithium 

Li. 

7 

Caesium * 

Cs. 

133 

^Magnesium 

Mg, 

24.4 

*Calcium 

Ca. 

40 

Manganese 

Mn. 

55 

*Carbon 

C 

12 

*Mercury 

Hg. 

200 

Cerium 

Ce. 

140.5 

Molybdenum 

Mo. 

95.5 

^Chlorine 

Cl. 

35.5 

Nickel 

Ni. 

58.6 

^Chromium 

Cr. 

52 

Niobium 

Nb. 

94 

Cobalt 

Co. 

58. C 

Nitrogen 

N. 

14 

^Copper 

Cu. 

63.2 

Norvvegium 

Ng. 

214 

Decipium 

Dp. 

159 

Osmium 

Os. 

198 6 

Didymium 

Di. 

146 

*OXYGEN 

O. 

16 

Erbium 

Er. 

165.9 

■^Palladium 

Pd. 

105.7 

'•Fluorine 

F, 

19 

■''’Phosphorus 

P. 

31 

Gallium 

Ga. 

68.8 

■^Platinum 

Pt. 

194.4 


THE CHEMICAL ELEMENTS. 


11 


TABLE OF THE CHEMICAL ELEMENTS— Continued. 


Name, 

Symbol. 

Atomic 

Weight. 

Name. 

Symbol. 

Atomic 

Weight. 

^Potassium 

K. 

39 

Tellurium 

Te. 

125 

Rhodium 

Rh. 

104 

Terbium 

Tb. 

148.8 

Rubidium 

Rb. 

85.3 

Thallium 

Tl. 

204 

Ruthenium 

Ru. 

104 

Thorium 

Th. 

233.4 

Samarium 

Sm. 

150 

*Tin 

Sn. 

118 

Scandium 

Sc. 

44 

Titanium 

Ti. 

48 

*Selenium 

Se. 

79 

Tungsten 

W. 

184 

*SlLICON 

Si. 

28.2 

*Uranium 

U. 

238.5 

^Silver 

Ag. 

107.7 

anadium 

V. 

51.3 

^Sodium 

Na. 

23 

Y tterbium 

Yb. 

172.8 

Strontium 

Sr. 

87.5 

Yttrium 

Y. 

89.8 

*SULPHUR 

S. 

32 

*Zinc 

Zn. 

65.3 

Tantalum 

Ta. 

182 

Zirconium 

Zr. 

90 


Symhols . — It is very convenient to use the first letter or 
letters of the name of an element to represent that element, 
instead of having to write the entire name. To such letters 
the name of symbols is applied. Many of the s^unbols, how- 
ever, are not derived from the ordinary names of the elements, 
but from their Latin names. 

Thus we have : 


Ordinary Name. 

Latin Name. 

Symbol. 

Antimony 

Stibium 

Sb. 

Copper 

Cuprum 

Cu. 

Gold 

Aurum 

Au 

Iron 

Ferrum 

Fe 

Lead 

PI urnbum 

Pb. 

Mercury 

Hydrargyrum 

Hg. 

Potassium 

Kali urn 

K. 

Silver 

Argentum 

Ag. 

Sodium 

Natrium 

Na. 

Tin 

Stannum 

Sn. 


Each symbol stands for a single atom of the element which 
it represents. When it is desired to indicate more than one 
atom, this is done by placing a small figure below and to the 
right of the symbol. Thus O, Oo, O4, represent one, two and 
four atoms of oxygen respectively. 

Atomic Weights . — The figures placed opposite to the sym- 
bols of the elements in the table above, represent the relative 
weights of the atoms of the various elements. Thus we 


12 


THE CHEMISTRY OF PHOTOGRAPHY. 


believe that an atom of oxygen is sixteen times, and an atom 
of mercury two hundred times as heavy as an atom of hydro- 
gen. Thus each symbol represents something more than the 
mere name of the element for which it stands. It also indi- 
cates a certain proportion by weight of that element ; indicat- 
ing, in fact, its atomic weight as well as its name. 

Classification of the Elements . — The mam division of the 
elements is into metals and non-metals. In the above table 
the names of the fifteen non-metals are printed in capital 
letters. There are three characters by which metals are com- 
monly distinguished, viz. : (1) the possession of metallic lus- 
ter, (2) they are good conductors of heat, (3) and good 
conductors of electricity ; these three properties are never 
found united in a non-metal. 

At the ordinary temperature of the air five of the elements 
are gaseous — oxygen, hydrogen, nitrogen, fiuorine and chlo- 
rine ; two are liquids — bromine and mercury ; and the 
remaining sixty three are solids. Every element has been 
liquefied by heat with the single exception of carbon. 

By means of great cold combined with great pressure, all 
the gaseous elements can be reduced to the liquid and even to 
the solid state. 

Thirty of the elements are designated rare, being found only 
in very small quantities. Oxygen is by far the most abundant 
element, constituting one-fifth of the air, eight-ninths (by 
weight) of water, and one-half of the solid crust of the Earth. 

In the substances more or less commonly employed in 
photography about one-half of the chemical elements are pre- 
sent. They are distinguished in the table by an asterisk (*). 

Chlorous and Basylous Elements . — The elements differ 
widely among themselves in their electrical properties. The 
metals are good conductors of electricity, allowing it to pass 
more or less freely ; while the non-metals such as sulphur, sili- 
con, etc., are non-conductors. It has been thought possible that 
it is the force of electricity which determines the chemical com- 
bination of the elements. Thus in the case of water we know 
that the hydrogen atoms are electro-positive as compared with 
the oxygen atoms which are electro-negative — for it is unlike 


THE CHEMICAL ELEMENTS. 


13 


kinds of electricity (positive and negative) which attract one 
another. There are eight of the elements — Chlorine, Bro- 
mine, Iodine, Fluorine, Oxygen, Sulphur, Selenium, and 
Tellurium — which are electro-negative when compared with 
any of the remaining sixty-two elements. These eight ele- 
ments are called chlorous or negative because chlorine is their 
type. The other sixty-two elements are called hasylous (or 
positive). 

The chemical union between any two or more chlorous 
elements is usually weah^ and such compounds are easily de- 
composed. On the contrary, the union of a chlorous with a 
hasylous element forms a compound which is more or less stable. 



CHAPTEK III. 


TERMS EMPLOYED IN CHEMISTRY. 

Chemical Formulm. — We have learned that the names of 
elements are represented by letters called symbols. The 
“symbols” of compound substances are formed by placing in 
juxtaposition the symbols of the elements of which the com- 
pound is composed. To such a collection or group of symbols 
the term chemicai formula is applied. 

Thus chloride of silver, which is composed of one atom each 
of the elements silver and chlorine, is represented by the 
formula AgCl. Every molecule of sulphuric acid contains 
seven atoms — two of hydrogen, one of sulphur, and four of 
oxygen — accordingly its formula is H3SO4. If we desire to 
represent more than one molecule, this may be done by plac- 
ing a large numeral in front of the formula. Thus 6 AgCl 
will stand for six molecules of silver chloride ; 4 H2SO4, for 
four molecules of sulphuric acid. 

Molecular Weights . — The weight of any molecule is found 
by adding together the weights of the atoms of the respective 
elements of which it is composed. Thus, supposing it is 
required to find the molecular weight of silver nitrate, AgNOg. 


Element. 

Atomic Weight. 

No. of Atoms. 


Ag. 

108. X 

1 = 

108. 

N 

14 X 

1 = 

14 

O 3 

16 X 

3 = 

48 


Molecular Weight, 

270 


Chemical Nomenclature. — The large number of names 
used in chemistry has led to the introduction of systems of 
framing these names which are very useful, because we then 
get as much information out of each name as possible. Still 
it must not be forgotten that there are exceptions to every 
rule, and many names have been applied to chemical sub- 
stances which later discoveries have shown to be incorrect. 


TERMS EMPLOYED IN CHEMISTRY. 


15 


All metals discovered in modern times have been given 
names ending in — um^ as magnesinm, chromium, etc. But 
here at once we meet witli exceptions, for the elements selen- 
ium and tellurium — thought to be of a metallic nature at the 
time of their discovery — have since been found to be more 
properly classed with the non-metals. 

Binary Compounds, — A compound containing two ele- 
ments only is called a binary compound. All the chemical 
names of such compounds end in — ide. Thus ‘‘red precipi- 
tate” — which is composed of oxygen and mercury — is known 
as mercuric oxide ; the name of the metal being formed into 
an adjective and placed first, while the non-metal has the affix 
— added to the first syllable of its name. Thus the 
name — mercuric oxide — tell us that the red precipitate is com- 
posed of mercury and of oxygen, and of these two elements 
only. But the same two elements frequently combine with one 
another in varying proportions, so forming two distinct binary 
compounds which it is necessary to distinguish from each 
other. In such a case the adjectival termination — ic — is 
assigned to the compound which contains the larger propor- 
tion of the non-metal; while to that which has less of the 
non-metal the termination — ous — is appended. Thus we have 
one atom of tin and two atoms of chlorine forming stannous 
chloride ; while one atom of tin and four atoms of chlorine 
form stannic chloride. 

It may even happen that the same two elements form more 
than two compounds. To that compound which has least of 
the non-metal we then give the prefix “ hypo-” ; to that which 
has most the prefix per-. Sometimes “ sub-” is used instead 
of “ hypo.” 

Thus we have — 

Nitrous Oxide — containing two atoms of nitrogen and one 
atom of oyxgen (UgO). 

Nitric Oxide — containing two atoms of nitrogen and two 
atoms of oxygen (N2^2)' 

And, — 

Nitric Per -oxide — containing two atoms of nitrogen and 
four atoms of oxygen (Il2^4)- 


16 


THE CHEMISTRY OF PHOTOGRAPHY. 


And again, — 

Hypo-chlorous oxide — containing one atom of oxygen and 
two atoms of clilorine (ClgO). 

And, — 

Chloric Per-oxide — containing four atoms of oxygen and 
two atoms of chlorine (CI2O4). 

Anhydrides. — There are certain oxides which when added 
to water, form / these oxides are termed anhydrides.’^ 
Thus we have carbonic anhydride, composed of one atom 
of carbon and two atoms of oxygen; and nitric anhydride, 
composed of two atoms of nitrogen and five atoms of oxy- 
gen, etc. 

Acids. — The term acid was at first applied to all substances 
having a sour taste like vinegar (which is dilute acetic acid). 
As used by chemists the term is now given to “ any compound 
containing one or more atoms of hydrogen, which are dis- 
placed when a metal is presented to the compound in the form 
of a hydrate.” Thus, if we mix hydrogen chloride (H Cl) 
with potassium hydrate (K H O), the metal potassium dis- 
places the hydrogen and forms potassium chloride (KCl)^ 
while the displaced hydrogen unites with the oxygen to form 
water (H^O). Hydrogen chloride is, therefore, called an acid. 
It is still true, however, that the majority of acids are sour- 
tasting substances. Another simple test for acids is the power 
which they possess of turning blue litmus red. Litmus is a 
vegetable coloring matter (obtained from a kind of lichen) 
which is used either in solution or on test-papers which have 
been dipped into litmus solution and then dried. 

Bases. — The term base is applied to certain compounds 
which, when they combine with acids, form salts. The best- 
known bases are (1) certain metallic oxides, such as potassium 
oxide (1^2 oxide (Zn O), etc. ; (2) certain metallic hy- 
drates, such as sodium hydrate (Ha H O); ( 3 ) certain other 
compounds, of which only ammonia (N II3) need here be 
named. The hydrates., or hydroxides, result from the com- 
bination of metallic oxides with water. Thus : — 


Barium oxide ^ water to for7U barium hydrate. 


TEKMS EMPLOYED IN CHEMISTRY. 


17 


Salts . — The chemical compound which results from the 
mutual action of an acid and a base, is called a salt ; e.g.^ 


Water of Crystallization . — The presence of water is neces- 
sary to the crystallization of many salts. When the water is 
driven off by heat, the crystal falls to powder. Each such 
salt unites with a certain definite quantity of water to form its 
crystals, and the union is an example of molecular combina- 
tion. In the absence of the water the salt is said to be in the 
anhydrous state. Thus silver sulphate (Agg SO 4) is naturally 
an anhydrous salt, not requiring the aid of water to form its 
crystals ; but the green crystals of ferrous sulphate consist oi 
Fe SO 4 combined with seven molecules of water. Its for- 
mula may be written Fe S 04 +7H2O ; or Fe SO4, THgO. At a 
temperature of 300 deg. C. all the water is driven off, and a 
white powder remains which consists of Fe SO 4 only. As 
chemists almost invariably sell such salts in their crystallized 
state, it is necessary — in calculating the amount of each con- 
stituent in a given weight of the salt — to take into account the 
water of crystallization. 


Ammonia 

{base) 


combines 

with 


hydrochloric acid to form 
{acid) 


ammonium 

chloride. 

{salt) 


CHAPTER lY. 


CHEMICAL LAWS AND THEORIES. 

Chemical Combination. — The introduction of the delicate 
form of weighing machine known as the balance into the 
study of chemical changes inaugurated a revolution in the 
science of chemistry. Among other things it proved that 
matter is indestructible — that when one form of matter dis- 
appears, an equal weight of some other form is produced. But 
it also showed that when the different elements, or compounds, 
enter into chemical combination with one another, they do so 
in fixed and unalterable pro])ortions by weight and by volume. 

Law of Constant Proportions. — Suppose we purchase a 
hundred specimens of pure nitrate of silver, each sample 
weighing 270 grains, prepared by as many chemists, living 
perhaps in different countries. Then analysis proves that 
every one of these hundred specimens is composed of 108 grains 
of metallic silver, 14 grains of nitrogen, and 48 grains of oxy- 
gen. It is the same with every other chemical compound, the 
proportions of each element of which it is formed are constant. 

Law of Multiple Proportions. — Although, in the same sub- 
stance, the elements which compose it hear a fixed proportion, 
by weight, to each other, yet it is frequently the case that the 
same elements can combine with each other in different pro- 
portions. But in each 'case the compound produced is distinct 
in its nature and properties. The binary compounds resulting 
from tlie chemical union of nitrogen with oxygen are an excel- 
lent example of this. Yo fewer than five such compounds are 
known which are shown in the following table : 


NAME. 

Formula. 

Containing 
parts of 
Nitrogen. 

Containing 
parts of 
Oxygen. 

Nitrous Oxide 

NgO 

28 

16 

Nitric Oxide 

NgO., 

28 

32 

Nitrous Anhydride 

NgOg 

28 

48 

Nitric Peroxide 

N 0 O 4 

28 

64 

Nitric Anhydride 

NoO, 

28 

80 


CHEMICAL LAWS AND THEORIES. 


19 


Each of these five compounds is composed of the same two 
elements, and yet they are five substances differing greatly 
in their properties. In the table it will be noticed that the 
varying proportions of the oxygen are all multiples of the 
atomic weight of oxygen — 16, and in all such cases the propor- 
tions are found to be simple multiples of some common factor. 

Atomic Theory , — It was such considerations as these, which, 
in the early part of the present century, led Dalton to the 
discovery of the atomic theory. This theory considers matter 
to be composed of extremely small parts called atoms^ which 
cannot by any means be divided : as, in fact, the word atom — 
that which cannot be cut — implies. 

The atoms do not all possess the same weights — those of one 
element are, as a rule, either heavier or lighter than those of 
another element ; thus an oxygen atom is sixteen times, and a 
nitrogen atom fourteen times as heavy as an atom of hydro- 
gen. We know this to be the case because a pint, say, of 
oxygen weighs sixteen times and a pint of nitrogen four- 
teen times as much as a pint of hydrogen. Now, we 
have every reason to believe that, under similar con- 
ditions, there are exactly the same number of atoms con- 
tained in equal volumes of these gases. Therefore each indi- 
vidual atom of O must be 16, and of N 14 times as heavy as 
each atom of H. 

Chemical combination consists in the union of atom with 
atom, to form molecules^ which are the smallest portions of 
matter capable of independent existence. Since an atom is 
‘‘ the smallest quantity of an element, by weight, which can 
enter into, or be expelled from a chemical compound,” we see 
why chemical combination, or decomj^osition, always takes 
place in accordance with some multiple of the atomic weights. 
It is not possible, for example, for 24 parts by weight of oxy- 
gen to unite with 28 parts of nitrogen, for (dividing each by 
their atomic weights) that would necessitate 1^ atoms of the 
former uniting with 2 atoms of the latter, and fractions of an 
atom cannot exist. 

Mixtures . — When two or more elements are simply mixed 
together, without the exercise of chemical force or affinity, each 


20 


THE CHEMISTRY OF PHOTOGRAPHY. 


element retains its own properties, and can be recognized bj 
these as a part of the mixture. Thus, let iron tilings be rubbed 
up in a mortar with some sulphur. A yellowish powder is 
obtained in which the particles of iron and of sulphur can still 
be plainly distinguished by the aid of a magnifying glass ; and 
by passing a magnet through the mixture the iron can be read- 
ily separated from the sulphur. 

Chemical Comjpounds . — In the formation of a true com- 
pound the chemical force is engaged, and the result of its 
action is the formation of a new substance, whose properties 
are usually very different from those of the elements which 
united to form it. Thus let the mixUire of iron and sulphur 
referred to above be strongly heated. The heat will stimulate 
the chemical affinity or liking of the iron for the sulphur, and 
the two elements will unite to form a compound which is 
known as sulphide of iron. This sulphide of iron is a blackish 
solid in which neither iron nor sulphur can be detected by the 
microscope, and it is quite unaffected by a magnet. 

Common and Scientific Names . — Many cheinical compounds 
are known by more than one name. It has frequently happened 
that a substance has been long in use before the discovery of its 
true chemical composition, and, consequently, before a correct 
name, according to the ideas of chemists, could be given to it. 
The old name, given to the substance before its true nature was 
known, often survives and is used for commercial purposes. 

In the following table the old and often commercial names 
of the various substances are given in alphabetical order, and 
opposite each is placed the name by which it is now more cor- 
rectly known in modern chemistry : 


Old or Commercial Name. 

Modern or Scientific Name. 

Formula. 

Alum. 

Aluminum Potassium ) 

Al,(SOd3, K^S- 

Sulphate. ) 

O4, 24 HgO. 
HNO3. 

Aquafortis 

Nitric Acid. 

Bichromate of Potash. 

Potassium Bichromate. 

KgCraO^. 

Blue Vitriol. 

Copper Sulphate. 

Cu SO4. 

Chalk (precipitated). 

Calcium Carbonate. 

Ca CO3. 

Common Salt or Rock Salt. 
Copperas. ) 

Sodium Chloride. 

Na Cl. 

Green Vitriol. >• 

Proto-sulphate of Iron, )| 

Ferrous Sulphate. 

1 

Fe SO4. 


CHEMICAL LAWS AND THEORIES. 


21 


Old or Commercial Name. 

Modern or Scientific Name. 

Formula. 

Corrosive Sublimate, or ) 
Bichloride of Mercury. ) 

Mercuric Chloride. 

HgCla. 

Hartshorn, or Spirits of i 
Hartshorn. ) 

Ammonia. 

N H3. 

Hyposulphite of Soda. 

Sodium Thiosulphate. 

Nag Sg O3. 

Lunar Caustic. 

Silver Nitrate. 

Ag N O3. 

Muriatic Acid, or Spirits of } 
Salt. J 

Hydrochloric Acid. 

HCl. 

Muriate of Ammonia. 

Ammonium Chloride. 

N H4CI. 

Oil of Vitriol. 

Sulphuric Acid. 

Hg SO4. 

Permanganate of Potash. 

Potassium Permanganate 

K Mn O4. 

Pyrogallic Acid. 

Pyrogallol. 

C6He03. 

Perchloride of Iron. 

Ferric Chloride. 

Fe, Clp. 

Red Prussiate of Potash. 

Potassium Ferricyanide 

K; Fe(CN)3. 

Sal-ammoniac 

Ammonium Chloride. 

N'H^ Cl, 

Soluble Gun Cotton. 

j Pyroxline, or Nitro- i 
i cellulose. f 


Spirits of Wine. 

Ethylic Alcohol. 

c, h, 0. 

Sulphuric Ether. 

Ether. 

C4 H,o 0. 

Sulphuretted Hydrogen. 

Hydrosulphuric Acid. 

Hs S. 

Tannin. 

Tannic Acid. 

C27 H03 O17. 

Wood Spirit. 

Methylic Alcohol. 

C H4 0. 

Yellow Prussiate of Potash. 

Potassium Ferrocyanide. 

K4 Fe fCN.)«. 

Aqua Retzia. 

Nitro-hydrochloric Acid. 

HCl -f HNO3. 

Sel D’Or, or Hypo-sulphite } 
of Gold. f 

Thiosulphate of Gold. 

Auo So O3. 

Chrome Alum. 

Chromium Po t a s s i u m 
Sulphate. 

Cr^ (S04)3,K2 
SO4 -F 24 OHo. 

Green Vitriol. 

Ferrous Sulphate. 

Fe SO47H2O. “ 

Blue Vitriol, 

Cupric Sulphate. 

Cu SO45H0O. 

Prussic Acid. 

Hydrocyanic Acid. 

H C N. 

Caustic Soda. 

Sodic Hydrate. 

Na HO. 

Caustic Potash. 

Potassic Hydrate. 

K H 0. 

Calomel. 

Mercurous Chloride. 

Hg,> Clo. 

Red Precipitate. 

Mercuric Oxide. 

HgO. 

Condy’s Fluid. 

Permanganate of Potash. 

K Mn O4. 

Borax. 

Sodic Borate. 

NaoB4O7l0HoO. 



CHAPTER Y. 


CHEMICAL MANIPULATIONS. 

Under this heading come the various operations used in 
chemistry, such as solution, distillation, etc., operations which 
the photographer will have to perform very frequently if he 
aspires to he something more than a drawing-room dabbler. 
We have no sympathy with the man who buys his developer 
ready-made, or perhaps uncaps the lens only, and then sends 
the plate to a professional photographer to be developed. 

Solution . — By solution we mean the process which takes 
place when a solid is converted into the liquid state by the aid 
of water or some dissolving agent. Thus when we put a piece 
of solid lump sugar into water we find that in a short time 
the sugar disappears. The sugar is no longer in the solid 
state, but becomes itself a liquid, occupying the pores or inter- 
spaces between the molecules of the water. That there is 
some force or mutual attraction acting between the solid and 
the dissolving liquid seems to be pretty certain, for whereas 
hyposulphite of soda, for example, is very soluble in water, it 
will not dissolve at all in alcohol. It is found that certain 
liquids have a peculiar attraction for a particular class of 
bodies ; thus water is au almost universal solvent for the 
class of bodies known as salts^ but will not readily dissolve 
such bodies as gums, resins and other colloidal substances, 
whereas ether, alcohol, chloroform, turpentine and other 
liquids belonging to the organic class of bodies are par- 
ticularly good solvents for these colloidal bodies, but are bad 
ones for salts, etc. The best way to dissolve a solid is to pow- 
der it in a mortar (Fig. 1) before bringing it into contact with 
the liquid ; this exposes a larger surface to the action of the 
liquid. The process of solution is also aided by heating the 
liquid. A given quantity of any liquid cannot at a given tem- 
perature dissolve more than a certain quantity of any solid. 


CHEMICAL MANIPULATIONS. 


23 


When it has dissolved as much as is possible, the liquid is said 
to be saturated. If more of the solid is then added it will 
remain in the solid state, usually lying at the bottom of the 
vessel in which the liquid is contained. Although a hot liquid 




will usually dissolve a solid more rapidly than a cold liquid, 
yet care must be used in its employment. Thus boiling water 
will partly decompose ferrous-sulphate, hyposulphite of soda, 
etc., and generally it may be said that though it is allowable 
to use the liquid warm it is safer in dissolving the ordinary 
chemicals employed in photography not to have it boiling. 
A good plan to dissolve a solid rapidly is to place it in a 
muslin bag, which is then suspended by a string so as to hang 
near the top of the bottle containing the liquid in which the 
solid is to be dissolved. Or a piece of paper may be cut out 
to tit in the neck of the bottle, and some pin-holes made 
in the centre ; it is then folded up, inserted in the neck 
of the bottle so that the lower end dips in the liquid, and 
the powdered solid is placed within it. These plans are 
advantageous because the saturated liquid is heavy and sinks 
to the bottom. 

Evaporation . — When a substance is in solution we use the 
process known as evaporation to obtain the substance in the 
solid state. By boiling, or evaporating, the liquid is driven 
off, leaving the solid substance behind. In evaporating solu- 
tions, vessels known as porcelain evaporating dishes (Fig. 2) 
are used, which are warmed uniformly by being placed on a 
sand-bath (a round iron or tin disJi containing sand), heated by 
a Bunsen burner ; the liquid should be occasionally stirred with 


24 


THE CHEMISTKY OF PHOTOGRAPHY. 


a glass rod during evaporation. The Bunsen burner (Fig 3) 
burns air and gas mixed, giving a hot, smokeless flame ; it 




Fig. 4. 


should be provided with a rose-top to spread out the flame 
when required. 

Precipitation . — When we mix two clear liquids together 
we often get a turbidity, caused by the formation of some 
insoluble matter ; the solid formed is known as a precipitate, 
and the process as precipitation. 

For example, when we mix a solution of silver nitrate and 
hydrochloric acid together, we get a white, curdy precipitate ; 
this is silver chloride, formed by the chemical reaction be- 
tween the two substances ; it is insoluble in either water or 
hydrochloric acid, and so it makes its appearance in the solid 
state. 


AgN 03 

+ 

HCl = 

AgCl + HNO 3 

Silver Nitrate. 

Joins 

with 

Hydro- to 

chloric for77i 
Acid 

Silver 

Chloride and Nitric Acid 

(insol.) 


For studying precipitation, test-glasses on feet (Fig 4) are 
very convenient ; or the thin glass tubes called test-tubes may 
be employed. 

Filtration . — When we wish to separate the precipitate from 
the liquid with which it is mixed, we use the process known 
as flltration. For this ]:)urpose we pour the whole upon some 
porous body which lets the liquid run through but will not 
permit the solid matter to pass ; the liquid which runs through 


CHEMICAL MANIPULATIONS. 


25 


is known as the filtrate. Porous paper, known as filter paper, 
is used for this purpose. Only matter suspended in water 
can be removed by filtration ; dissolved matter cannot be 
filtered off. 

Filter paper is usually sold in circular pieces ; fold the paper 
in half, then fold again ; it is now a quarter-circle ; now open 




Figs. 5, 6, 7. 



it so as to form a hollow paper cone ; this cone is fitted into a 
glass funnel (Figs. 5, 6, 7) and gently moistened wdth water 



before passing the liquid through which it is desired to filter. 
A filter-stand (Fig. 8) is useful to hold the funnel, and the fil- 
trate may be received in a test-glass or in a beaker (Fig. 9). 


26 


THE CHEMI8TKY OF PHOTOGRAPHY. 


Decantation . — For heayy precipitates, and for precipitates 
which, fall to the bottom rapidly, the process of decantation 
may be employed. The beaker containing the precipitate is 
allowed to stand for some time until all the solid matter has 
settled to the bottom. The clear liquid is then carefully noured 



Stout glass beakers, ^ and A; thin beakers for holding hot liquids, C and D. The latter 
are arranged in sets, or nests, fitting one within the other. 


off, aud distilled water added to the precipitate ; the whole is 
shaken up, and the precipitate is again allowed to settle, 
and .the clear water poured off. This washing is repeated 
several times, until the precipitate is completely freed from 
the adhering solution ; this is known as washing by decan- 
tation. 

Glass vessels shaped like Fig. 10 are very convenient for this 
sort of work. 



Distillation . — The process called distillation is 
used to purify liquids ; or to separate liquids from 
solids, or from other liquids which boil at different 
temperatures. 

Distillation is effected by boiling the liquid in a 
retort, cooling or condensing the vapor given off by 
Fig 10 . means of a condenser, and collecting the condensed 
liquid in a vessel known as a receiver ; the impurities and 
solid matter remain behind in the retort. Ordinary water 
contains several impurities in the shape of dissolved solid mat- 
ter, and this renders it unfit for many chemical purposes ; it 
therefore has to be distilled to render it pure. 

Water may be distilled on a small scale by placingthe liquid 



CHEMICAL MANIPULATIONS. 


27 


in a glass flask (Fig. 11), the neck of which is connected with 
a long glass tube which is surrounded by an outer jacket ol 
glass or tin through which cold water must be continually 
kept flowing. This piece of apparatus is known as Liebig’s 



condenser. The end of the glass tube is placed within a flask 
known as a receiver, because it receives the condensed liquid. 
The water in the retort is heated by a Bunsen burner ; the 
steam given ofl passes down through the condenser ; it is there 



cooled and condensed, and the distilled whaler is collected in 
the receiver. Where large quantities are required, the still is 
best made of copper, and the spiral tube, or worm,” of tin 
(Fig. 12). 

Fractional Distillation . — The process of distillation may be 
made use of to separate or fractionate liquids, which have 
different boiling points. Suppose we have a mixture of two 


28 


THE CHEMISTKr OF PHOTOGEAPHY. 


liquids whose boiling points differ rather widely, say, water and 
alcohol. Water we know boils at 212 deg. F., whereas 
alcohol boils at 173 deg. F. ; we place the mixture in a retort 
and heat gently. A vapor is given off and condenses in the 
receiver, and if we have a thermometer connected with the 
inside of the retort, we shall see that it stands at about 173 F. 
This shows us that the alcohol only is boiling, and that our 
‘‘ distillate” is chiefly alcohol ; after a time, the thermometer 
will slowly rise, and steam, or water- vapor, will be given off. 
Change the receiver and collect the condensed water. By this 
means we have more or less completely separated the alcohol 
from the water, and by repeating the process on each of the 
distillates we can get nearly pure samples of water and of 
alcohol. 


Manipulation of Glass, Glass-Blowing, Etc. 

The photographer has often to flt up and make certain 
pieces of apparatus which require a slight knowledge of the 
manipulation of glass, and if he has not had previous experi- 
ence he will usually waste a lot of time and glass before he 
gains his desired ends. We therefore propose to give a few 
hints which should enable any one with a little practice to 
become fairly proflcient in the art of manipulating glass. 

Kind of Glass to he Employed . — There are two sorts of glass 
tubing — hard glass and soft glass tube, the latter is the kind 
most generally used. There are also two varieties of soft glass 
tubing, one known as lead glass and the other as soda glass. 
Tlie lead glass tubing is easy to work, but has the unpleasant 
property of blackening in the reducing flame, so that soda 
glass is, on the whole, the best sort to use (Fig 13). 

O OO ooooo 

Fig. 13. 

Blow-Pipe , — For some things an ordinary Bunsen burner 
will be sufficient ; but it will be found that for many purposes 
a blow-pipe of some kind will be indispensable. The common 


CHEMICAL MANIPULATIONS. 


29 


mouth blow-pipe known as Black’s is very useful, but requires 
a little practice (Fig 14). It is used in conjunction with a 
Bunsen burner ; the white or gas flame of the Bunsen is em- 
ployed, and the nozzle of the blow-pipe is placed just inside the 



bottom part of the flame, and a gentle blast will give you a 
flame possessing great heating power. The blow-pipe flame con- 
sists of two parts, an inner and an outer cone ; a point a little 
beyond the end of the inner cone will be found to be the 
hottest part of the entire flame. For blow-pipe purposes a 
continuous blast is necessary. This is obtained by inflating the 




Fig. 16. 


cheeks and breathing through the nose. Ailing the cheeks when 
required from the lungs. Fletcher’s ‘‘ Ilerepath ” blow-pipe 
with foot blower (Figs. 15 and 16) is a most useful piece of 
apparatus ; but for ordinary purposes it may be dispensed with. 

To Cut Glass Tubing . — Glass tubing is usually cut by 


30 


THE CHEMISTHY OF PHOTOGKAPHT. 


means of a three-cornered or triangular file ; a small file with 
fine serrated edges will be found best. For thin tubes all that 
is necessary is to draw the edge of the file across the part you 
wish to cut ; then take hold of the tubing on both sides of 
the cut with the hands (the file mark being uppermost) and, 
pulling with a slight downward tendency (Fig. 17), the tube 



will be found to break clean at the file cut. For very thick and 
strong tubing it will be found necessary to file all around the 
desired part ; then apply a piece of red-hot glass or a hot 
poker to the cut and it will crack easily. 

Bending Glass , — A fiame is required that will cover a large 
surface of glass, and the ordinary fish-tail and batVwing fiames 
are the best for this purpose ; of the two the fish-tail is prefer- 
able. FTever attempt to bend glass tubing in a Bunsen fiame, 
as it is next to impossible to do it properly. To bend a piece of 
tubing, you hold it with one hand on each side of the particu- 



Fig. 18. 


lar part you wish to operate upon, and then place the part near 
the top of the fish-tail fiame, and gently rotate the tube in order 
to heat it uniformly over a space of two or three inches (Fig. 
18). After a time you will feel the glass soften ; then take it 


CHEMICAL MANIPULATIONS. 


31 


out of the flame and bend it to the desired angle. The glass 
will now be covered with a layer of soot ; allow it to cool with 
the soot on it, so as to anneal the glass ; that is, prevent any 
strains being set up by unequal cooling. Lastly, round ofl the two 
rough ends of the tube by heating them in the Bunsen flame. 

To Draw Out a Glass Jet . — For this purpose a blow- pipe 
flame is required, although if the glass is thin it may be done 
in a Bunsen flame. The glass is held in both hands and a 
blow-pipe flame is caused to impinge on the part to be drawn 
out. Eotate the glass until soft, then take it out of the flame 
and pull gently, not suddenly ; you will then have two pieces 
of glass joined by a flnfe capillary tube (Fig. 19). Cut the 



Fig, 19. 


fine tube at the part required and just melt the edges in the 
flame, and anneal by placing in a crucible of dry sand, which 
allows the glass to cool slowly. All glass should be annealed 
after heating ; if this is not done it will generally be found 
to crack on cooling ; and unannealed glass also flies to pieces 
when heated. 



To Seal or Close Glass Tuhing . — Proceed precisely as if 
you were going to draw out a jet ; then heat the capillary tube 
at the part nearest the glass, and draw out ; rotate the sealed 
end in the blow-pipe flame until soft (Fig. 20), and then blow 


32 


THE CHEMISTRY OF PHOTOGRAPHY. 


very gently down the tube until the glass at the end is 
properly rounded ; anneal thoroughly afterwards. 

A piece of glass tubing about twelve inches long, sealed up 
at both ends, will be found useful as a stirring rod for stirring 
up solutions. It is not so liable to go through the bottom of 
a glass beaker as the same length of glass rod. 





CHAPTEK VI. 


PREPARATION OF GASES. 

As an introduction to the study of practical chemistry, the 
beginner cannot do better than endeavor to prepare the ele- 
mentary gaseous bodies — hydrogen, nitrogen and oxygen — and 
study their properties when in the free or uncombined state. 
He will thereby learn lessons in manipulation which will be 
available to him in all photographic processes. 

Oxygen. 

Symbol, O. Atomic weight 16. 

Oxygen exists free in the atmosphere, but it is there mixed 
with about four times as much of nitrogen ; the composition 
(by bulk) of pure air being — 


Oxygen 21 

Nitrogen 79 


100 

Oxygen was first obtained by itself in 1774, by Dr. Priest- 
ley. He heated some red precipitate (oxide of mercury. 



HgO) in a glass tube and noted that this substance was de- 
composed into a gas (oxygen), which issued from the tube, 
while metallic mercury remained behind (Fig 21). 

HgO = Hg + O. 


34 


THE CHEMISTRY OF PHOTOGRAPHY. 


The gas so prepared was found to possess many remarkable 
properties. It was named oxygen producer of sour 

things) by the great French chemist Lavoisier, because he be- 
lieved it to be a necessary part of all acids. 

Oxygen is most conveniently prepared by intimately mix- 
ing together five parts, by weight, of potassium chlorate with 
one part of black oxide of manganese. Each pound of the 
chlorate will produce nearly four cubic feet of oxygen. 
As an experiment, place a couple of ounces of the mixture in 
a thin glass flask (Florence flasks, in which oil is imported, 
answer very well), fitted with a cork and delivery tube, the 
other end of which dips under the water in a pneumatic 
trough. Heat the flask gently and oxygen gas will be given 
ofl in abundance. It may be collected by placing the end of 
the delivery tube underneath the mouths of bottles or cylin- 
ders filled with water, and inverted in the trough. 

KCIO3 = KCl. + 3,0. 

Potassium Potassium Oxygen. 

Chlorate Chloride 

• 

The potassium chloride remains in the flask mixed with the 
black oxide of manganese, which itself undergoes no change 
(Fig. 22). 



Oxygen gas is colorless, tasteless, transparent and inodorous. 
It combines eagerly with nearly all the other elements, the 


PREPARATION OF GASES. 


35 


process of combination being known as oxidation. Tt is the 
oxygen in the air which sustains tlie burning of all our candles, 
fires, and gas-jets, for, although itself incombustible, it is the 
great supporter of combustion. Substances which burn in 
the air burn far more brilliantly in pure oxygen. Thus, if a 
piece of charcoal (which only smoulders away in the air) be 
fastened by wire to an iron rod, heated and introduced into a 
bottle full of oxygen it blazes up and emits showers of sparks. 
The result of this combination is the formation of an oxide of 
carbon commonly known as carbonic acid gas. 

C + 02=: CO 2 

Carbon unites with Oxygen to form Carbonic Acid Gas. 

The metal magnesium has a great affinity for oxygen, and 
when burned in a large vessel of the gas gives a flame of 
dazzling brilliancy. 

Mg + o = MgO. 

Magnesium combines with Oxygen to form Oxide of Magnesium. 

Or we may mix the powdered metal with some substance 
rich in oxygen, as chlorate of potash, gun-cotton, etc. The 
mixture will burn almost instantaneously when a light is 
applied, and this “ flash-light ” is now largely used in winter 
for taking portraits, etc. 

Oxygen is the most abundant of all the elements. It forms 
(by weight) more than one-flfth of the atmosphere, eight- 
ninths of water, and one-half of the rocks which compose the 
crust of the Earth. 

Several plans have been devised for obtaining oxygen from 
steam, or from the atmosphere. Ten or twelve years ago 
Tessie du Motay used manganate of soda, which absorbed oxy- 
gen when a current of superheated steam was passed over it, and 
became converted into permanganate. The latter substance 
was then strongly heated, when it gave up its absorbed oxy- 
gen, returning to the state of manganate, which could be used 
over and over again. 

This method, however, did not prove the commercial success 
which was expected. 


36 


THE CHEMISTRY OF PHOTOOEAPHY. 


In 1886 M. Brins commenced working on a large scale a 
method of extracting oxygen from the air by heating barium 
oxide (BaO) in retorts. At a temperature of about 900 deg. 
Falir. this substance absorbs oxygen and is converted into 
barium peroxide (BaOg). But when the heat is raised to 
1400 deg. Fahr. the peroxide is decomposed as follows: 

BaOg = BaO + O. 

Barium peroxide yields barium oxide and oxygen. 

Oxygen gas prepared by this process, and compressed into 
steel cylinders, is now sold in London at fourpence per cubic 
foot. 

Hydrogen. 

Symbol, H. Atomic weight, 1. 

Hydrogen gas is the lightest of all known substances, and 
hence is frequently used for filling balloons. Hydrogen is 
colorless, tasteless, transparent and without smell. In these 
properties it resembles oxygen ; but it differs from that ele- 
ment in being highly inflammable, burning with a pale blue 
flame. The most abundant source of hydrogen is water, of 
which it forms one-ninth by weight ; but it is also an ingre- 
dient of many other substances. 



Hydrogen gas is most conveniently prepared from sulphuric 
acid by acting upon it with zinc (Fig. 23). Cut the zinc into 
small pieces and place them in a Woulffs two-necked bottle 
fitted with a thistle-funnel and a deli very -tube. Dilute the 


PREPARATION OF GASES. 


37 


sulphuric acid with eight times its weight of water and pour 
it upon the zinc. A violent bubbling is seen, and hydrogen 
gas escapes through the delivery-tube. It may be collected in 
bottles over the pneumatic trough. 

H2SO4 + Zn = ZnSO^ -f Hg 

Sulphuric Acid and Zinc produce Sulphate of Zinc and Hydrogen. 

A mixture of hydrogen and air (and especially of hydrogen 
and oxygen) is explosive ; and in lantern entertainments where 
the ‘‘mixed gases” were used, many serious accidents have 
been caused by the violent combination of the two elements. 

Hydrogen appears to have been known to the alchemist 
Paracelsus, in the sixteenth century ; but its properties were 
first scientifically studied by Cavendish in 1781. 

Nitrogen. 

Symbol, N. Atomic weight, 14. 

Nitrogen resembles both oxygen and hydrogen in being 
colorless, transparent, tasteless and inodorous. But it differs 
from oxygen in not being a supporter of combustion (a lighted 
candle goes out immediately when placed in a bottle of nitro- 
gen gas), and from hydrogen in not being inflammable. 



Nitrogen exists free in the air, of which it forms nearly 
four-fifths ; combined with other substances it occurs in the 
bodies of animals and plants, and it is also an ingredient of 
many chemical compounds— ammonia (NH3) for example. 


38 


THE CHEMISTRY OF PHOTOGRAPHY. 


Nitrogen is readily obtained from the air by removing the 
oxygen with which it is mixed. 

Place a small piece of dried phosphorus in a little porce- 
lain crucible floating on the water in a pneumatic trough ; 
place a bell- jar over the saucer, and ignite the phosphorus by 
touching it with a hot wire, putting in the stopper of the jar 
immediately the wire is removed. The phosphorus burns 
vigorously, combining with the oxygen to form dense white 
fumes, which are oxide of phosphorus (Fig. 24). These 
fumes dissolve in the water under tlie bell-jar, and we thus 
remove all the oxygen from the air within the jar, the water 
rising up one-hfth the height of the jar to take its place, show- 
ing that oxygen forms one-fiftli of the atmosphere ; the re- 
maining gas is nitrogen. Introduce a lighted candle into the 
bell-jar and it at once ceases to burn. Nitrogen is a very inert 
element, not combining readily with the other elements. In 
conjunction with hydrogen and oxygen, however, it forms a 
powerful acid. Nitric Acid (UNO 3 ). 

Until the last month of the year 1877, the gaseous elements 
Oxygen, Hydrogen and Nitrogen were known as the perma- 
nent gases j because they never had been liquefied — much less 
solidified. But in that year the Continental experimentalists 
Cailletet and Pictet succeeded — by using great cold and tre- 
mendous pressure — in reducing all of these substances first to 
the liquid and then to the solid state. 


CHAPTER YII. 


BOOKS, APPARATUS, CHEMICALS. 

Chemistry is a very wide subject, and those who wish to 
study it fully will find the following books very useful. 

I. As a general introduction to the science, Roscoe's Les- 
sons in Elementary Chemistry,'’ j^nblished by Macmillan, 
4rS. 6A ; Sexton’s Stockhardt and Heaton’s “ Princi^Dles of 
Chemistry,'’ Bell, 5^9. ; Thorpe’s “ Inorganic Chemistry,” 
2 vols., Collins, 6s. ; P'owne’s and Watts’ ‘‘Chemistry,” Yol. 
I., Inorganic, 9^. ; Yol. IL, Organic, lO^., Churchill. 

II. Books treating of the qualitative analysis of substances ; 
i.e., that branch of chemistry which tells us how to discover 
simply of what chemical elements any given substance is com- 
posed : Thorpe and Muir’s “ Manual of Qualitative Analysis,” 
Longmans, 6s. 6d. : Clowes’ “ Qualitative Inorganic Analysis,” 
Churchill, 7^. 6d. 

III. Books treating of quantitative analysis, by which we 
determine not only of what elements any given substance is 
comj^osed, but also how much, by weight, of each element is 
contained in it : Thorpe’s “ Quantitative Analysis,” Longmans, 
4^. 6d. ; Fresenius’ “ Quantitative Analysis,” Yol. I. 
Churchill, 15<§. 

lY. Books of reference* : “ Chemistry,” by Roscoe and 
Schorlemmer, six vols., 18^9. and 21.5. each, Macmillan ; Watts’ 
“ Dictionary of Chemistry,” nine vols., £15 2s. 6d., Longmans 
(a new edition of this most valuable book is now appearing). 

Y. There are also several hooks dealing with the theories 
upon which our modern chemistry is based, which the student 
will find deeply interesting : Cooke’s “ The Hew Chemistry,” 
Kegan Paul, 6s . ; Tilden’s “ Chemical Philosophy,” Long- 
mans, 6s\ 6d. ; Muir’s “ Principles of Chemistry,” Cambridge 

*The Appendix, by Professor Ehrmann, to the recently published “ Photographic 
Instructor,” is a valuable work of reference on the nature and use of the various chemicals 
and substances employed in photographic practice —Editor Photographic Times. 


40 


THE CHEMISTEY OF PHOTOGEAPHY. 


Press, 15^. ; Meyer’s “ Modern Theories of Chemistry,” Long- 
mans, 18^. 

Chemical Appaeatus 

Chemistry learned from books alone is all but valueless. 
True chemistry is, above all things, an experimental, observa 
tional and inductive science. The apparatus and chemicals 
contained in the following lists will enable any one to go 
through the ordinary practical course in chemistry as laid 
down in the text-books, and to conduct qualitative analyses, 
h^or quantitative analysis, a delicate balance in glass case (Fig. 
45) will be the principal additional article required. 



Fig. 45. 

List of Chemical Appaeatus. 

2 hard glass flasks, fitted with safety thistle funnels and leading-tubes, 
arranged for the preparation of hydrogen, carbonic acid, chlorine 
gases, etc. (Fig. 25.) 



BOOKS, APPARATUS, CHEMICALS. 


41 


2 hard glass flasks, with leading-tubes, for the preparation of oxygen, 
laughing-gas, etc. 

1 flask-holder, for the hand. 

Sheet-iron retort, for oxygen. 

Japanned tin pneumatic trough, with side-shelves. (Fig 26.) 



Fig. 25. 



Fig. 26. 


Metal spirit-lamp with double wick, and ring to support flasks, or. 
where gas is obtainable, a Bunsen’s gas-lamp instead of the spirit- 
lamp. 

1 iron tripod, with sand-bath dish, (Fig. 27.) 

1 gas receiver, capacity one pint, fitted with brass cap, stop-cock, bladder 

and ferrule, and brass jet for burning hydrogen. 

3 gas receivers, one quart capacity, one plain and two stoppered. 

2 earthenware trays for removing gas receivers from pneumatic trough 

when filled. 

3 ground-glass plate covers, for gas receivers. 

Deflagrating jar, one pint capacity, with ground edge, brass cap and spoon 
for phosphorus, sulphur, etc. 

1 taper-holder. 

Strong glass tube, for exploding the mixture of hydrogen and oxygen. 



Fig 27. 



2 goldbeaters skin balloons for hydrogen. 

Mouthpiece, for inhaling laughing-gas from a bladder or gas-bag. 
Conical brass blow-pipe. 

6 in. platinum wire. 

Piece 2 in. X 1 in. platinum foil. 

1 test-tube stand, 24 holes. (Fig 28.) 


42 


THE CHEMISTRY OF PHOTOGRAPHY. 


36 test-tubes, 6-in. x ^-in. 
24 " 5-in. X 3^-in. 

1 test-tube basket. 


1 » " holder. (Fig 29.) 

2 boiling-tubes. 

5 test-tube brushes. 

1 set of 5 spouted beakers. 

1 each Bohemian flasks, 2-oz., 4-oz., 8-oz., 16-oz., and 30-oz. 
1 each Berlin crucibles, IJ-in. and 1-^-in. 

1 n " basins, 2^-in., 3^-in. and 4-in. 

1 « « funnels, 1^-in. and 2-in. 

3 glass funnels 234-inch. 

1 glass funnel, 3-inch. 

1 black-wood funnel stand. 

2 quires filter paper. 

1 set of 4-filter cutters. 

1 tripod stand, 5-in. 

Iron retoit stand, with 3 rings. (Fig. 30.) 



Fir,. 31. 



Fig. 30. 


2 pieces iron gauze, 5-in. square 

1 sand-bath, 5-in. (Fig 31.) 

6 watch-glasses, 3-in. diam. 

2 lbs. glass tube. 

1 lb. « rod. 

lb. combustion tube. 

G feet black india-rubber tube 1 round 

2 n . 


2 acid-funnels, 18-in. (Fig. 32.) 

6 dozen assorted corks. 

2 Woulflfe s bottles, 20-oz., two necks. 
1 stoppered retort, 2-oz. (Fig. 33.) 

1 set of 3 cork-borers. 

1 triangular file, 4-in. 


» O " 

34-in. 1 square-flat « 8 " 

1 Bunsen’s burner, with blow-pipe jet, star support and chimney. 
1 rose burner for same. 

1 pair brass crucible tongs, 8-in. (i ig. 34 
1 porcelain mortar, 4-in., and pestle. 

1 steel spatula, 5-in. (Fig. 35 ) 

6 boxes litmus test-papers. 


BOOKS, APPAEATUS, CHEMICALS. 


43 


2 deflagrating spoons. (Fig 3G.) 

Wash-bottle. (Fig. 37.) 

1 thermometer, enameled, 300° Cent. 

1 each stoppered flasks, graduated to deliver 250, 500 and 1,000 c.c. 
1 each pipettes, graduated to deliver 25, 50 and 100 c.c. 

1 pipette, 10 c.c. graduated in yijj. 



Fig. 32. Fig. 34. 

1 pair watch-glasses and clip. 

1 small stoppered weighing-bottle. 

1 tile, 6-in., glazed both sides. 




1 packet Swedish filter paper, 4^-in. 

2 Berlin crucibles and covers. 

1 Geissler burette, 100 c.c. in 


(Fig. 38.) 


44 


THE CHEMISTRY OF PHOTOGRAPHY. 


1 stand for same. 

6 beakers, 20-oz., and covers. 



Fig. 39. 


Fig. 40. 


1 glass desiccator. 

1 pair of small scales with weights, to pack in box. (Fig. 39.) 






2 graduated glass-measures (Fig. 40). 1 solid-flame gas-burner (Fig. 41). 

1 glass jar, with stop-cock, to hold distilled water, etc. (Fig. 42.) 


1 lb. acid hydrochloric, pure. 
1 lb. " " com’l. 

1 lb " nitric, pure. 

1 lb. " sulphuric, com’l. 


Chemicals. 

^ oz. cadmium metal. 

^ oz. cobalt nitrate. 

^ oz. manganese sulphate. 
1 oz. microcosmic salt. 


BOOKS, APPAKATUS, CHEMICALS. 


45 


1 Ib. acid, sulphuric, pure. 

2 oz. » oxalic. 

1 lb. ammonia .880. 

4 oz. ammonium chloride. 

2 oz. " nitrate. 

■| lb. iron sulphide. 

2 oz. asbestos, picked. 

2 oz. barium chloride. 

4 oz. calcium chloride, dried. 

^ lb. wood charcoal. 

^ lb. copper turnings. 

4 oz. iron filings. 

1 oz. lead acetate. 

1 oz. litmus. 

2 oz. magnesium sulphate. 

1 lb. manganese binoxide. 

^ lb. marble. 

1 oz. plaster of Paris. 

^ oz. phosphorus. 

2 oz. potassium bichromate, pure. 

2 oz. chlorate. 

4 oz. " nitrate. 

2 oz. " ferrocyanide. 

2 oz. // ferricyanide. 

4 oz. glycerine. 

4 oz. fluor spar. 

^ oz. silver nitrate. 

1 lb. caustic soda. 

J oz. sodium metal. 

4 oz. sodium acetate. 

1 pint methylated spirit. 

•I lb. sulphur, roll. 

lb. zinc, granulated. 

3 packets of labels. 

2 oz. acid arsenious. 

^ oz ammonium phosphate. 

2 oz. » oxalate, pure. 

^ oz. bismuth, metal. 

2 oz. borax. 


2 oz. tartaric acid, cryst. 

^ oz. nickel sulphate. 

1 oz. potash alum. 

^ oz. strontium nitrate. 

4 oz. zinc, purified. 

1 lb. acid, acetic. 

1 oz. " boracic. 

^ oz. '/ molybdic. 

2 oz. ammonium chloride, pure. 

1 lb. " sulphide. 

2 oz. " carbonate. 

1 oz. barium hydrate. 

1 oz. chloroform. 

4 oz. ether, sulphuric, methylated. 
^ oz. potassium bromide, 
oz. » iodide. 

2 oz. sodium carbonate, dry, pure. 

2 oz. " phosphate. 

1 oz. copper sulphate. 

1 oz. Iceland spar. 

2 oz. iodine, resublimed. 

1 oz. pure iron (piano-forte) wire. 
^ oz. mercury chloride. 

^ oz. lead nitrate. 

1 oz. potassium chromate. 

1 oz. " permanganate. 

1 oz. " sulphate. 

2 oz. sodium chloride. 

4 oz. 1 hyposulphite. 

1 oz. tin, granulated. 

1 oz. uranium nitrate. 

^ oz. zinc sulphate. 

1 lb. copper turnings. 

2 oz. ferric chloride. 

1 lb. paraffine wax. 

lb. starch. 

1 lb. calcium oxide (quicklime). 

■| pint alcohol. 

1 quart methylated spirit. 


If tlie student lives at a distance from any reliable dealer, 
it will be well to lay in duplicates of all the glass appa- 
ratus, and four times the weight of each chemical named 
above. In the absence of a gas supply a couple of sjoirit- 
lamps and half a gallon of methylated spirit will be required 
as a substitute. 


46 


THE CHEMISTEY OF PHOTOGKAPHY. 


Chemicals in the solid state should be kept in wide-mouthed 
corked (or, better, stoppered) bottles. Chemicals in 
the liquid state should be kept in narrow-mouthed 
bottles (Fig. 43) having ground-glass stoppers (the 
stoppers should be lubricated with a little vaseline). 
Acids, alcohol, ether, and ammonia should especially 
never be kept in corked bottles. 

All chemicals should be kept in a cupboard, under 
lock and key. Many of them are dangerous poisons, 
and numerous fatal accidents have occurred through their 
being carelessly kept. Every bottle should be distinctly 
labeled, and the label coated first with size and then with thin 
varnish. 




Fig. 43. 



CHAPTER YIIL 


TREATMENT OF RESIDUES. 

Residues SJtould le Collected . — Certain of the sub- 
stances used by the photographer — the compounds of gold 
and silver for example — are very costly ; and it is fortunate 
that for the production of each picture only a very small 
quantity of these valuable articles is required. Put although 
the amount of these precious metals actually retained in each 
negative or print is very small, yet for the perfect production 
of the picture it is necessary that there should be present — at 
the commencement of the operation — a much larger quantity. 
How, with many photographers, the proportion not used — the 
residue — goes down the sink and is lost. This is a pity, be- 
cause it is an easy thing to recover the valuable portion of 
such residues, and thereby to make photography more profitable 
to the professional and a less expensive recreation to the 
amateur, while the task of recovery will teach more than one 
lesson of value. 

Residues to he Preserved , — Of the various reagents em- 
ployed by the photographer, five at least are of sutficient value 
to require their collection and preservation witli a view to sub- 
sequent treatment. These are : (1) The salts of gold ; (2) the 
salts of silver; (3) salts of platinum; (d) alcohol, and (5) 
potassium oxalate. 

How TO Collect Residues. 

Certain vessels must be set aside — one for each residue — so 
that six or seven receptacles will be wanted altogether. The 
best shape is conical, as the solid matter then sinks more 
rapidly to the bottom, not having the same chance of adher- 
ing to the sides. A plug, or tap of some kind, should be 
placed near the bottom of each vessel, so that the clear liquid 
above can be drawn ofi from time to time. 


48 


THE CHEMISTKY OF PHOTOGRAPHY. 


For the gold residue a glass vessel is best, while earthen- 
ware answers for the silver ; even casks, tarred inside, are used 
bj many. Every solution should be emptied into its proper 
vessel immediately it is done with, and the dish which con- 
tained it washed out. It is comparatively easy to clean a dish 
or vessel just after use, but when the dregs are allowed to dry, 
they are far more difficult to remove. 

Recovery of Gold from Residues. 

Gold residues — the spent toning-baths — should be collected 
in conical glass precipitating-jars. Such jars can be obtained 
of any size up to a gallon, and the size of jar to be employed 
must, of course, be regulated by the extent of the photog- 
rapher’s operations. Although the bath may refuse to 
tone any more prints, yet it still contains a considerable 
percentage of the amount of gold which was added to 
it to commence with, and also a larger amount of silver 
chloride, vrashed off the as yet unfixed prints which have been 
soaked in it. 

When the gold residue jar is nearly full, add a little sul- 
phuric, and then a rather larger quantity of hydrochloric acid 
— say an ounce of the two to a quart of solution. ]^ow stir 
in gradually a saturated solution of ferrous sulphate, until it 
ceases to produce the least precipitate. A black deposit is 
formed, which consists of metallic gold mixed with carbonate 
and oxide of iron. This precipitate is in such a fine state of 
division that it sinks very very slowly and must be allov/ed a 
couple of days to go to the bottom. 

Now very carefully pour away — or syphon off — the super- 
natant clear liquid and collect and dry the residue. In all 
ordinary cases it is better to keep all residues of gold and sil- 
ver until they have reached a certain bulk, and then send them 
to a respectable refiner, who will extract the precious metals 
with far greater certainty and less expense than any individual 
not in that special trade. For this purpose it is only necessary 
to scrape out the moist residue with a spoon, place it in a por- 
celain dish or crucible — or in one of the enameled iron dishes 
now so common — and dry it in an oven. 


TREATMENT OF RESIDUES. 


49 


Reduction of Gold Residues. 

If the j^liotograplier, however, wishes to see for himself 
how much gold he can actually exti*act, he must add water 
acidulated with sulphuric acid to the residue, and stir it up 
well, then allow to settle, and repeat the process ; and, lastly, 
wash it well twice with pure water. Lastly, let the residue be 
dried and placed in a little ‘‘assay pot” lined with borax, 
along with some fusion mixture.. The pot must be strongly 
heated in a furnace, when a button of mixed gold and siL er 
will be found at the bottom. Add to this button three times 
its weight of silver, melt the whole in a plumbago crucible, 
and pour the liquid metal into cold water. Dissolve the gran- 
ulated metal in warm dilute nitric acid, when the silver will 
be removed in tiie form of nitrate, and the gold will be left 
as a brown powder, which may then be converted into chloride. 

Recovery of Silver from Residues. 

The “ silver waste ” is derived from at least three sources, 
and the waste from each source had better be treated separately. 

First we have the trimmings of the sensitive paper. 

It is always best to trim the prints before toning, as no gold 
is then used up in toning the margins, and, moreover, the 
prints are less likely to tear in the subsequent washing. All 
such trimmings should be kept in a dry box or basket. Then 
there are lilter papers, through which solutions of silver have 
been passed, and the bits of blotting paper used to absorb the 
drainings from sensitized paper ; all these should be dried and 
added to the rest. 

When the receptacle for paper is full the contents must be 
burned — a handful at a time — on a large iron tray placed on 
three or four bricks. Each pound of paper cuttings should 
yield about an ounce of drab-colored ash. An ordinary fire 
grate may be used for the burning if it is first carefully 
cleaned out. The paper should be lighted at the top^ and 
fresh paper added little by little ; otherwise the strong draught 
may carry part of the fine ashes up the chimney. 

Next comes an important source of silver — the wash-water 


50 


THE CIIEMISTEY OF PHOTOGRAPHY. 


from silver prints. Every one knows how milky the first 
two or three waters appear in which prints are washed before 
toning. This milkiness is due to chloride of silver washed 
out of the paper ; and it is certain that many thousand dollars’ 
worth of silver are in this way literally thrown away every 
year. 

The first three — or even the first six — wash-waters should be 
placed in a large earthenware vessel or tarred tub, and with 
each a little commercial hydrochloric acid must be added, 
which will rapidly precipitate the silver chloride, causing it to 
settle at the bottom as a white mud. Many workers add com- 
mon salt instead of the acid, and this answers fairly well ; the 
objection to it is that silver chloride is soluble in a strong solu- 
tion of common salt, and as this substance is so cheap, too 
much of it is frequently added, the result being that some of 
the silver is poured away in solution. Before the clear liquid 
is thrown away a drop of hydrochloric acid, or a solution of 
common salt should be added to a glassful of the liquid ; if 
any cloudiness then appears it shows that the whole of the sil- 
ver chloride has not been precipitated. As the vessel fills, the 
clear upper portion may be drawn off from time to time by a 
tap or syphon, or by simply lading it out or pouring it off. 
When the deposit at the bottom has reached a considerable 
amount, all the liquid should be poured off and the deposit 
stirred up with clean water. Lastly, pour the water away and 
remove the residue with a spoon, placing it in a dish in the 
oven to dry. 

Another source of silver is in the old hyposulphite of soda 
fixing-haths, both of negatives and prints. These should be 
poured as they are done with into a separate vessel. They 
contain silver in the form of a double salt — the hyposulphite 
of silver and sodium. If several strips of zinc are kept hang- 
ing in the vessel much of the silver will be deposited on the 
zinc through the electrical action which is set up. This silver 
adheres loosely to the zinc as a dark powder, and may readily 
be brushed off. To secure the remainder of the silver in the 
solution, some liver-of-sulphur (potassium sulphide) should be 
added, which will precipitate the silver as black silver sulphide. 


TREATMENT OF RESIDUES. 


51 


Tlie latter operation should be conducted in the open air, as 
offensive fumes of sulphuretted hydrogen are given off. The 
black silver sulphide may be scraped out, dried and sent to the 
refiner ; or it may be reduced by heating to a red heat in a 
fire-clay crucible with saltpetre on a clear fire. The contents 
of the crucible are removed, washed with water and filtered, 
when the pure silver is left on the filter. It can then be dis- 
solved in nitric acid to form silver nitrate. 

Another residue which contains silver is spoiled gelatine 
emulsion — which may be boiled with a little sulphuric acid so 
as to be rendered incapable of setting, and added to the hypo 
residue. 

A neat method for the recovery of metallic silver from 
silver chloride is to melt the chloride in a ])orcelain dish over 
a Bunsen burner, and then to insert in the liquid one end of a 
piece of platinum wire about six inches in length. When the 
silver chloride cools it will hold this end of the wire firmly. 
Now attach the other end of the wire (by means of a bind- 
ing screw) to a strip of amalgamated zinc and immerse the 
whole in a vessel of dilute hydrochloric acid. 

After a few minutes the mass of silver chloride will be seen 
to become streaked with gray porous metallic silver, and in a 
few hours the whole of the chloride will be reduced. The 
spongy mass of silver remaining can now easily be detached 
from the porcelain dish, and must be washed to free it from 
acid, and then dried. To obtain the silver as a button of 
bright metal the grayish mass must be fused before the blow- 
pipe in a bone-ash cupel with a little lead until the latter 
metal becomes oxidized and is absorbed by the cupel, leaving 
pure silver behind. 

Recovery of Silver in the ^¥et Way . — Another method of 
obtaining silver from silver chloride is to decompose the salt 
by electricity. The residue containing the silver chloride may 
be placed in any large-mouthed earthenware vessel, in the cen- 
tre of which must be placed a porous cell such as is generally 
employed in the construction of galvanic batteries. The 
porous cell must be filled with water acidulated with a little 
sulphuric acid, and in it must be placed a rod of zinc which 


52 


THE CHEMISTRY OF PHOTOGRAPHY. 


has been amalgamated (coated with mercury) ; a copper wire 
is soldered to the zinc, and to the other end of the wire a sil- 
ver plate (a large spoon will do) is attached, and this is made 
to dip into the silver solution. An electric current is immedi- 
ately produced, and the metallic silver slowly settles upon the 
silver plate, which may then be taken out and the particles of 
silver rubbed oif and well washed. 

Results of Recovering the Residues of the Precious 

Metals. 

One of our leading photographers — Mr. Valentine, of Dun- 
dee — latelj' published the results which he obtained from the 
collection of the gold and silver residues employed in his 
extensive business. In a given time he used £691 10s. Od. 
worth of nitrate of silver. Of this he received from the refiner 
as the value of the ashes of sensitized paper, £104 (>s. 3d. ; the 
residues obtained from the washing of prints were worth 
£178 10s. 3d. ; and the value of the old hyposulphite of soda 
fixing-baths £193 16s. 4d. Thus the value of the silver resi- 
dues was £476 7s. 3d., so that more than two-thirds of the 
silver employed was actually recovered. 

Of gold used to the value of £274, there was recovered from 
the spent toning-baths £101 14s. 3d. The refiner’s charge, for 
reducing both gold and silver to the metallic state, was 
£24 10s. 9d., and Mr. Valentine adds, ‘‘ I have never been able 
to do it so cheaply myself.” 

From these considerations, it is evident that care in looking 
after residues may make a considerable difference in the profits 
of a large business. 

Platinum Residues. 

Metallic platinum is worth about half as much as gold, so 
that workers of the platinotype process will find it worth their 
while to collect all their ^datinum residues. The solution 
with which the Platiiiotyjie Company coat their paper con- 
tains about sixty grains of platinum salt to the ounce ; and as 
only one-tenth of the metallic platinum present is used in 
forming the picture, it is evident that the remaining nine- 


TREATMENT OF RESIDUES. 


53 


tenths is left for collection by the careful worker. When the 
used baths amount to a quart or two they should be heated in 
a large beaker over a sand-bath to near the boiling point, and 
a saturated solution of ferrous-sulphate added at the rate of 
half a pint to each quart of the residue. This will precipitate 
the platinum as a black powder, which will speedily sink to 
the bottom, and may be collected, washed and dried. The 
liquid remaining is ferrous-oxalate, and may be added to the 
oxalate residues. 

Another method is to evaporate all the platinum residues to 
dryness. Burn the paper, and mix all the ashes, etc., in a clay 
crucible with fusion mixture, and heat strongly. After half 
an hour spongy platinum will be formed. Wash this well 
with water, and dissolve it in aqua regia (three parts HCl to 
one of HAO3). Evaporate the solution to dryness, add water, 
and then ammonium chloride. A yellow precipitate of the 
double chloride of platinum and ammonia is formed. Filter 
this off, wash it with methylated spirit, dry and ignite 
in a porcelain crucible, when pure metallic platinum will be 
obtained. 

Oxalate Residues. 

The comparatively high first cost of the ferrous-oxalate 
developer is sometimes urged as an argument against its use, 
the cost of the neutral potassium-oxalate which is employed in 
making it being thirty-two cents per pound. But if the 
used developer is preserved until a sufficient quantity has 
accumulated, it is not difficult to prepare the potassium-oxalate 
from it again at a nominal cost. The solution to be treated 
must be placed in a large glass vessel and potassium-carbonate 
gradually added until a precipitate ceases to be produced. By 
filtering, the powdered catbonate of iron which has been 
formed can be removed, and the filtrate should be perfectly 
clear. Oxalic acid must now be added to the filtrate until 
litmus paper shows that the solution is neutral, after whieh it 
can be evaporated down till crystals begin to appear. It is 
then a saturated solution of neutral potassium-oxalate, and is 
ready for use again. 


54 


THE CHEMISTRY OF PHOTOGRAPHY. 


Alcohol. 

Although less alcohol is now used than formerly in the prep- 
aration of collodion, owing to the introduction of gelatine 
dry plates, yet, on the whole, a very much larger quantity is 
employed in photography, as it is now used in preparing and 
precipitating gelatine emulsion, for hardening and drying 
negatives, and in compounding certain developers. 

For ordinary work it is sufficient to add potassium carbonate 
to the dilute alcohol. The potassium salt combines with the 
water and separates from the alcohol, which is left on top and 
can be syphoned off. By heating the potassium carbonate it 
can be freed from the water and is then ready for use once 
more. 

If, however, it is necessary that the alcohol should be jpure^ 
as in that required for the manufacture of collodion, the above 
plan will not answer, and resort must be had to distillation 
with quick-lime. 


CHAPTEE IX. 


TABLE OF CHEMICAL ELEMENTS AND COMPOUNDS 
COMMONLY EMPLOYED IN PHOTOGRAPHY. 





Mole- 


NAME. 

FORMULAS. 


cular 

Weight 

PRICE. 

Acetic Acid. 

C,H,0, 


GO 

^0 20 lb 

Albumen. 

i Ng.SP. 



1 00 lb 

Asphaltum. 




15 lb 

Alcohol (Ethylic Alcohol). 

C,H,0 


46 

40 lb 

Aldehyde (Acetyl Aldehyde). 

CoH^O 


44 

1 00 lb 

Aluminium-Ammonium-Sulphate ( 

) 

AL(S0J3+(N 
HJ2S04 + 24 

1 

906.8 

10 lb 

(Ammonia Alum). | 

\ 

OH. 


Aluminium Potassium-Sulphate ( 

) 

AL(S6J3 + K.,S 


768 

10 lb 

(Common Potash Alum). j 


04 + 240 H.' 


Aluminic Nitrate. 

i Al 2 (N 03 )y h 

( 16011., 


326 

2 00 1b 

Aluminic Sulphate. 

j Al2(SO/)3-f- 
] 180H, 

666 

10 1b 

Ammonia. 

NH3 


17 


Ammonium. 

NH4 


18 


Hydrate. 

NH4HO 


85 

15 ib 

» Bichromate. 

(NH4)2Cr20, 


252 

1 50 lb 

» Bromide. 

NH^Br 


98 

65 lb 

H Chloride. 

NH4CI 


531 

12 lb 

u Iodide. 

NHJ 


145~ 

40 oz 

Nitrate. 

(NHJ NO3 


80 

30 lb 

<! Sulphydrate. 

(NH4) HS 


51 

45 Ib 

Arsenic Bromide. 

AsBr, 


315 

65 oz 

Barium. 

Ba. 


137 

4 00 grm 

/' Chloride. 

Ba Cl2-f20H2 


345 

241b 

" Nitrate. 

Ba(N03)o 


261 ‘ 

15 lb 

Benzole (or Benzine). 

CeHe 


78 

20 lb 

Cadmium. 

Cd 


i 112 

20 oz 

'/ Bromide. 

CdBrg-h4HoO 


344 

26 oz 

" Iodide. 

Cd G 


366 

50 oz 

Calcium Bromide. 

Ca Br., 


200 

15 oz 

" Chloride. 

Ca CI2V6H0O 


219 

10 lb 

" Iodide. 

Ca I2 


294 

50 oz 

Camphor. 

C.oH.^O 


152 

35 lb 

Canada Balsam. 



45 lb 

Caoutchouc. 




2 50 lb 

Carbolic Acid (Phenol). 

C0H3O 


94 

50 lb 

Carbonic Acid Gas. 

COo 


44 

50 lb 

Castor ( )il. 




30 lb 

Chloroform. 

CHC13 ” ’ 


ii4 

60 lb 

Chlorinated Lime (Bleaching Pow- ) 
der or Calcium Hypochlorite). ( 

CafOCl) Cl 


127 

10 lb 


56 


THE CHEMISTEY OF PHOTOGEAPHY. 


TABLE OF CHEMICAL ELEMENTS, Y.TQ.— Continued. 




Mole- 


NAME. 

FORMULAS. 

cular 

Weig^ht 

PRICE. 

Chromium Potassium Sulphate. 

j Cr,(S 04 ) 3 ,K 2 ) 
\ SO 4 + 24 H 2 O ) 

998 

.... 

Citric Acid. 

CeH^O. + H^O 

210 

$0 75 1b 

Collodion. 



1 50 1b 

Copper Sub-Bromide. 

CuBr 

143 


" Bromide. 

CuBr^ 

223 

60 oz 

» Nitrate. 

j Cu(NOa )2 + ) 

1 3H,0 f 

241 

65 lb 

" Sulphate. 

CUSO 4 + 5 H 2 O 1 

249 

10 lb 

Cyanogen. 

C., N.,(or Cy^) 

52 

.... 

Ether (Sulphuric Ether). 

C^H^oO 

74 

75 lb 

X Methylated. 

C^H.O'* ■■ 

46 

— 

« Methylic. 

... 

Formic Acid. 

CH 2 O 2 

46 

1 50 lb 

Gallic Acid. 

C.H^O, 

170 

1 401b 

Gelatine. 



2 00 lb 

Glycerine. 

LsHgOg 

92 

25 lb 

Gold. 

Au 

196 

50 grm 

" Cyanide. 

Au (CN) 

222 

2 50 grm 

» Sodium Chloride. 

)NaAuCL-f 1 

} 2 H 2 O f 

;J97 

30 grm 

V Trichloride. 

Au CI 3 

8021 

14 00 oz 

» Sodium Hyposulphite. 

j AuNa 3 S 40 g+ 1 
i 2 H 2 O ) 

525 


Gum Arabic. 


801b 

X Tragacanth. 
Hydriodic Acid. 

HI 

128 

50 lb 

Hydrochloric Acid. 

HCl 

361 

id ib 

Hydrocyanic Acid (Prussic Acid). 

HCN (or HCy) 

27'' 

1 00 lb 

Hydrobromic Acid. 

H Br 

81 

25 oz 

Hydrofluoric Acid. 

HF 

20 

2 00 lb 

Hydrokinone. 

c,u,o. 

110 

1 00 oz 

Hydrosulphuric Acid (Sulphuret- ( 

HoS 

34 

50 oz 

ted Hydrogen). f 


Hydroxyl. 

HgO., 

34 

50 lb 

Hyd roxylamine. 

NH 3 O 

33 


Hydrochloride. 

NH^lOHjCl 

691 


Hypochlorous Acid. 

HCl 0 

521 

1 25 grm 

Iridium Tetrachloride. 

Ir CI 4 

335 

Iron Acetate. 

jFe(C2H302)2 ) 

i +4H.O f 

246 

25 oz 

r Ammonium Citrate. 


70 lb 

,, V Sulphate. 

j FeS 04 (NH 4)2 ) 
) S 04 + 6 Ho 0 f 

392 

14 lb 

X Nitrate {Ferric Nitrate). 

Fe 2 (N 03)6 

484 

1 50 ib 

X Oxalate {Ferric Oxalate). 

Feo(Co04)3 

376 

X Sulphate {Ferric Sulphate). 

Fe2(S04)3+9HoO 

566 

10 ib 

X Bromide {Ferrous Bromide). 

Fe Br» + 6 H 0 O 

324 

25 oz 

X Chloride {Ferrous Chloride). 

Fe-CE. 

127 

20 lb 

X Iodide {Ferrous Iodide). 

FeI .2 + 4H20 

382 

40 oz 

X Nitrate {Ferrous Nitrate). 

Fe(N 03 ) + 6 H 20 

283 

60 ib 

X Perchloride {Ferric Chloride). 

FcoCL 

325 

X Protosulphate Sulphate 

FeS04+7H20 

278 

10 lb 


TABLE OF CHEMICAL ELEMENTS, ETC. 


5T 


TABLE OF CHEMICAL ELEMENTS, Y.'YZ.—Coniinued. 




1 Mole- 

i 

NAME. , 

FORMULAS. 

cular 

Weight 

1 PRICE. 

Kaolin {China Clay), 



i $0 10 lb 

Lead Acetate {Sugar of Lead). 

Pb(C,H 30,)2 

324 

25 lb 

" Chloride. 

PbCL 

242 

1 75 lb 

» Ferrocyanide. 

3 Pb 2 Fe(CN )6 + ) 
\ dHgO f 

678 

20 oz 

n Nitrate. 

PblNOgL 

330i 

; 20 lb 

Lithium Bromide. 

LiBr 

87 

40 oz 

» Iodide. 

Lil +3HoO 

188 

80 oz 

Magnesium. 

Mg 

24 

' 60 oz 

« Bromide. 

MgBr, 

i 184 

50 lb 

// Carbonate. 

VlgCOg 

1 84 

35 lb 

« Iodide. 

MgL 

i 278 

72 oz 

« Nitrate. 

Mg(N 03 ) 2 -f- 6 H 20 

256 

1 00 lb 

" Sulphate. 

MgS04 +7HoO 

246 

10 lb 

Mercury. 

Hg 

200 

80 lb 

« Bichloride. 

HgCL 

271 

75 lb 

" Sub-chloride. 

Hg,CL 

471 


1 Monoxide. 

HgO 

216 


Naphtha. 



35 ib 

Nitric Acid. 

HNO, 

63 

12 lb 

Nitro-Hydrochloric Acid. 

HNOg-t-SHCl 



Nitrous Acid. 

HNOs 

47 

20 ib 

Oxalic Acid. 

C 3 H 204 + 2 H .,0 

126 

20 lb 

Ozone. 

IO 3 

48 


Phosphoric Acid. 

IH 3 P 04 

98 

70 ib 

Platinum. 

IPt 

194 

9 00 oz 

" Tetrachloride. 

jPtCl4+5H.,0 

426 

5 00 oz 

Potassium. 

K 

39 

3 00 oz 

n Bichromate. 

K.Xr.,0- 

294 

15 lb 

« Bromide. 

KBr 

119 

45 lb 

« Carbonate. 

K,C 03 

138 

14 lb 

/' iTlorate. 

KCIO 3 

1224 

25 lb 

« Chloride. 

KCl 

74i 

40 lb 

!' C'yanide, 

K(CN)(or KCy) 

65 

55 lb 

X Ferricyanide. 

K 3 peCy 3 

329 

80 lb 

X Ferrocyanide. 

K 4 FeCyg + 3HoO 

422 

30 lb 

X Fluoride. 

KF 

58 

2 00 lb 

// Hydrate. 

KHO 

56 

60 lb 

" Iodide. 

KI 

166 

30 oz 

X Nitrate. 

KNO 3 

101 

12 lb 

X Nitrite. 

KNO.> 

85 

75 lb 

Oxalate. 

K 0 C..O 4 4-5HoO 

212 

25 lb 

Permanganate. 

Kr^MnoOs 

316 

30 lb 

X Sulphate. 

K 0 SO 4 ' 

174 

15 lb 

X Silver Cyanide. 

KAg(CN )3 

200 

25 lb 

" Sulphide. 

K^S 

110 

90 lb 

" Sulpho-cyanide. 

KS(CN) 

97 

1 15 lb 

Prussian Blue. 

Fe 4 (FeCy 3)3 

860 

60 lb 

Pyrogallic Acid. 

C.H 3 O 3 

126 

35 oz 

Pyroxyline. 

L 1 8 E 0 2(N0 0 isC 1 5 

846 

4 00 lb 

Silver. 

Ag 

108 

1 25 oz 

" Acetate. 

CgHgAgO, 

167 

2 50 oz 

X Ammonio-nitrate. 

[AgN 03 + 2 NH 3 

204 

.... 


58 THE CHEMISTRY OF PHOTOGRAPHY. 

TABLE OF CHEMICAL ELEMENTS, Continued. 





Mole- 




NAME. 

FORMULAS. 

cular 

PRICE. 




Weight 



Silver Bromide. 

AgBr 

188 

$1 

50 oz 

u 

Carbonate. 

AgsCOa 

276 

3 

70 oz 

„ 

Chloride. 

AgCl 

143i 

1 

35 oz 


Citrate. 

C 6 H 50 ,Ag 3 

513 


• . . » 

» 

Fluoride. 

AgF 

127 


.... 

w 

Hyposul phite. 

AggSgOg 

328 



II 

lodate. 

AgI 03 

283 


. . . 

n 

Iodide. 

Agl 

235 

1 

75 oz 

n 

Nitrate. 

AgN 03 

170 


75 oz 

II 

Nitrite. 

AgNO, 

154 

2 

50 oz 

II 

Oxide. 

Ag^O 

232 

1 

50 oz 


Sub-oxide. 

Ag^O 

448 



II 

Phosphate. 

Ag 3 P 04 

419 

2 

00 oz 

II 

Sodium Hyposulphite. 

AgNaS 303 + 2 H 20 

279 



II 

Sulphate. 

Ag^SO^ 

312 

1 

75 oz 

II 

Sulphide. 

Ag^S 

248 

3 

00 oz 

Sodium. 

N a 

23 


50 oz 


Acetate. 

( NaCgH309+ } 

} f 

136 


40 lb 

// 

Bicarbonate. 

HNaC 03 

84 


10 lb 


Borate (Borax). 

( Na.B, 0^ + 10 ) 
1 H 2 O f 

382 


15 1b 

„ 

Bromide. 

NaBr 

103 


60 1 b 

1, 

Carbonate. 

Na 2 CO 3 -f- 10 HgO 

286 


10 lb 

„ 

Chloride. 

NaCl 

584 


401b 

„ 

Hydrate. 

NaHO 

40“ 


15 lb 

II 

Hypochlorite, 

NaOCl 




" 

Hyposulphite. 

jNa.So03+5 ( 

( H 3 U f 

248 


10 lb 

„ 

Iodide. 

Nal 

150 

3 

90 lb 

„ 

Nitrate, 

NaN 03 

85 


12 lb 

„ 

Silicate. 

NaoSi(33 

302 


50 lb 

„ 

Sul phite. 

Na;S 03 + 7H20 

252 


25 lb 

n 

Tungstate. 

Na 2 W 042 + H 20 

330 


35 lb 

Starch. 

CeHio05 

162 


15 1b 

Strontium Chloride. 

SrCL+GH^O 

2661 


20 Ib 

Sugar (Sucrose). 

L 12 H 22 O 11 

342 



Sulphuric Acid, 

H 3 SO 4 

98 


ioVh 

Sulphurous Acid. 

H 2 SO 3 

82 


20 lb 

Tannic i^Ncid (Tannin). 

C 14 H 10 O, 

322 

1 

45 lb 

Tartaric Acid. 

0 4 H 6 0 g 

150 


50 lb 

Uranium Nitrate. 

(U0.3(N03)3+ ) 

1 6 H 20 ) 

504 


75 oz 

Vanadium. 

V 

51 

22 

00 grm 

Water, 

H 30 

18 


15 gal 

Zinc 

Bromide. 

ZnBrg 

225 


23 oz 

1, 

Chloride. 

ZnClo 

136 


75 1b. 

II 

Iodide. 

Znl 3 “ 

I 319 


50 oz 


The prices given above are those of a leading American firm. 


CHAPTER X. 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


Acetic Acid. 


Formula, CoH^Og : Combining weight, 60 . 

Vinegar — which is weak acetic acid — was tlie only acid 
known to ancients. When alcoholic liquors — as wine or beer 
— are oxidized by fermentation, acetic acid is produced. 
Immense quantities of “wdne vinegar” are made in France 
from the poorer classes of grape-juice, while “ malt- vinegar ” 
is now largely made in England. The oxidation of the 
alcohol is effected by a minute organism — the vinegar-plant” 
— which exists in countless myriads in the liquid, and which 
absorbs oxygen from the air and then transfers it to the 
alcohol. 

Strong acetic acid is obtained by adding sodium car- 
bonate to vinegar, and then distilling the sodium acetate so 
formed with sulphuric acid. 

When wood is heated in a retort an impure kind of acetic 
acid distills over. This is knowm 2i^ jjyroligneo'us acid \ it is 
largely employed in commerce, and pure acetic acid can be 
readily prepared from it. 

Pure acetic acid is a colorless liquid which solidifies at 62 deg. 
F., and forms large transparent crystals. Hence it is known as 
glacial (or ice-like) acetic acid. Beaufoy’s” acetic acid is a 
weaker form of the same substance, containing only thirty per 
cent, of the true acid. 

Acetic acid mixes readily with water ; it has a strongly acid 
reaction, and a pungent smell and taste. It is very corrosive, 
blistering the skin. Acetic acid is a good solvent for many 


60 


THE CHEMISTKY OF PHOTOGEAPHY. 


substances, including camphor and resins. As impurities 
abound in vinegar, that substance is not fit for use in ordinary 
chemical operations. But even the so-called pure acid fre- 
quently contains traces of sulphurous and hydrochloric acids, 
which may be detected by adding a little of the glacial acid to 
a solution of nitrate of silver. The mixture should remain 
colorless after it has been allowed to stand for several hours. 
Acetic acid often contains free sulphuric acid as an impurity. 
This may be detected by mixing with the acetic acid a little 
powdered starch. Boil, cool and add potassium iodide. A 
blue coloration indicates that free sulphuric acid is absent. 
But a blue coloration shows the ■presence of this acid, the 
starch being converted into glucose. 

Acetic acid is used in the developer for collodion plates ; 
also in printing upon bromide paper to prevent discoloration. 

Albumen. 

Albumen is a very complex substance which exists in many 
modifications in both animals and plants. The albumen con- 
tained in white of egg (about 12 per cent.) may be taken as a 
typical example, and its formula may be written ^ 3 

0 18^2 3 SB; but this can only be taken as giving a general 
idea of the chemical composition of this complex substance. 
The presence of sulphur in egg-albumen is proved by the 
well known blackening which silver egg-spoons undergo, and 
which is due to the formation of silver sulphide. 

Egg-albumen also contains traces of sodium, chlorine and 
calcium phosphate, and the whole has a cellular structure. 
By beating up the white, the cells are broken, and the mineral 
impurities can then be removed by the addition, first, of basic 
lead acetate, and then of carbonic acid gas. 

When egg-albumen is spread out in a thin layer and put in 
a warm place, it dries up to a yellow gum-like substance, 
which is the state in which it is usually sold by chemists. 

This solid albumen is insoluble in alcohol or ether, but 
dissolves slowly in warm water, the solution being hastened 
by the addition of a little common salt. One part of solid 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


61 


albumen dissolved in seven parts of water yields a solution 
equal in strength to ordinary white of egg. 

When liquid albumen is raised to a temperature of 150 deg. 
F., it begins to coagulate, and if the liquid be strong it is 
converted into a solid whitish mass; if dilute, it is simply 
rendered turbid. This coagulated albumen is insoluble in 
water. 

Albumen is precipitated from its solutions by alcohol, by 
nitrate of silver, and by hydrochloric acid. Albumen is also 
present in blood, as serum- alhumen. It is a good example of 
a colloid body ; never crystallizing under any conditions. It 
may be separated from crystalloids by dialysis. The sulphur 
contained in albumen is one of the possible causes of the almost 
universal fading of prints on albumenized paper. 

Albumen is largely used in photography, being universally 
employed to give a brilliant surface to the paper upon which 
photographic prints are produced (it was first used for this 
purpose by Fox Talbot, about 1852). Albumen was the first 
substance used to form a film in which the sensitive salts of 
silver could be spread out upon a glass plate, the albumen 
process being invented by N^iepce de St. Victor, in 1847. This 
process is still employed for ordinary transparencies. 

Alcohol (Ethylic Alcohol). 

Formula, CglTgO: Combining weight, 46. 

The term alcohol — spirits of wine — was originally applied 
only to the volatile inllammable liquid which is produced by 
the fermentation of sugar. The term has been extended, how- 
ever, by chemists to numerous other bodies whose properties 
more or less resemble those of the liquid from which the class 
takes its name. 

Ethylic alcohol, or spirits of wine, may be prepared either 
from the sweet juices of such fruits as the grape, or from a 
solution of cane-sugar, or from various kinds of grain, aud the 
potato. By fermentation — due to the action of a minute 
plant— the sugary liquids are converted into alcohol and car- 


62 


THE CHEMISTEY OF PHOTOGEAPHY. 


bonic acid gas. The latter escapes, while the alcohol, mixed 
with water, ivmains. 

To separate the spirit from the water, dist Illation 
to, the liquids being heated in a still. Now, the boiling-point 
of alcohol is onlj 173 deg. F. (that of water being 212 deg. F.), 
so that the more volatile alcohol passes away through the tube 
or worm of the still, leaving the water behind. Jhit a certain 
quantity of water (about 10 per cent.) passes over with it, so 
that it is not possible to obtain pure or absolute alcohol by this 
method alone. The “ rectified spirit” obtained by distillation 
repeated two or three times, usually has a specific gravity of 
.820 to .830 The next step is to add to the rectified spirit 
some substance, such as quick-lime or anhydrous copper 
sulphate, which will combine with and absorb the last traces 
of water. After distillation with this substance the absolute 
alcohol of commerce is obtained ; but even this contains one- 
half per cent, of water. If, for experiment, it is necessary that 
even this small fraction should be removed, it may be done 
by adding a little metallic sodium and again distilling. 

Absolute alcohol is a colorless, mobile liquid, has a pleasant 
smell, burning taste, and highly intoxicating properties. It 
burns readily with a bluish flame, and its vapor forms an 
explosive mixture with air. At freezing point (32 deg. F.) 
the specific gravity of alcohol is .806, and at 59 deg. F. it is .793. 
It quickly absorbs moisture from the air, so that it must be 
kept in glass bottles with carefully ground glass stoppers. To 
determine the amount of alcohol in any aqueous solution, an 
instrument called the hydrometer is generally employed. This 
consists of a closed glass or metal tube, upon which a scale is 
marked, and which is made to float upright by means of a 
loaded bulb at one end. As alcohol is less buoyant than water 
this instrument sinks deeper and deeper as the percentage of 
alcohol in the mixture increases, and by noting the point on 
the scale which is level with the surface of the fluid, the per- 
centage of alcohol present can be determined by reference to 
printed tables constructed for the purpose. 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


63 


Tables Showing the Amount of Alcohol, by Weight, 
Present in any Mixture of Alcohol and Water. 
Temperature, 60 ^ F. 


Specific 

rjravity. 

Alcohol 

by 

Weight 

Specific 

gravity. 

Alcohol 

by 

Weight. 

Specific 

gravity. 

Alcohol 

by 

Weight 

.793 

100 

.882 

66 

.954 

32 

.798 

99 

.884 

65 

.956 

31 

.800 

98 

.886 

64 

.958 

30 

.803 

97 

.889 

63 

.959 

29 

806 

96 

.891 

62 

.961 

28 

.809 

95 

.893 

61 

.962 

27 

.812 

94 

.896 

60 

.964 

26 

.814 

93 

.898 

59 

.965 

25 

.817 

92 

.900 

58 

.966 

24 

.820 

91 

.903 

57 

.968 

23 

.823 

90 

.905 

56 

.969 

22 

.825 

89 

.907 

55 

.970 

21 

.828 

88 

.909 

54 

.971 

20 

.831 

87 

.912 

53 

.973 

19 

.833 

86 

.914 

52 

.974 

18 

.836 

85 

.916 

51 

.975 

17 

.838 

84 

.918 

50 

.976 

16 

.841 

83 

.920 

49 

.977 

15 

.843 

82 

.923 

48 

.979 

14 

.846 

81 

.925 

47 

.980 

13 

.848 

80 

.957 

46 

.681 

12 

.851 

79 

929 

45 

.983 

11 

.853 

78 

.931 

44 

.984 

10 

.856 

77 

.933 

43 

.985 

9 

.858 

76 

.935 

42 

.987 

8 

.860 

75 

.937 

41 

.988 

7 

.863 

74 

.939 

40 

.990 

6 

.865 

73 

.941 

39 

.991 

5 

.868 

72 

.943 

38 

.993 

4 

.870 

71 

.945 

37 

.994 

3 

.872 

70 

.947 

36 

.996 

2 

.875 

69 

.949 

35 

.998 

1 

.377 

68 

.951 

34 

1.000 

0 

.879 

67 

.953 

33 




The high price of pure ethjlic alcohol renders it an expen- 
sive liquid to use in any quantity. To meet the wants of 
manufacturers and men of science, a mixture (known as 
‘‘ methylated spirit ”) of ordinary alcohol with 10 per cent, of 
methyl alcohol is allowed to pass free of duty. It cannot, 
however, he sold without a license. Methyl alcohol is also 
known as wood-spirit, because it is obtained, along with acetic 
acid, from the dry distillation of wood. This admixture pro- 


64 


THE CHEMISIEY OF PHOTOGRAPHY. 


duces a nauseous taste, but for most purposes it answers as 
well as the pure alcohol. 

Besides tliis methjlated spirit,” or methylated alcohol,” 
another liquid, known as methylated finish,” is sold, which 
contains a small quantity of resin. This renders it quite unfit 
for use in pliotography. 

The photographic uses of alcohol are either as a solvent ora 
drier. Owing to the affinity of alcohol for water, it soon 
extracts that liquid from emulsions, wet gelatine plates, or 
soaked carbon tissue. Alcohol is also an ingredient of several 
developers. F usel oil, a substance which is mostly amyl alcohol, 
is frequently present in ethylic alcohol as an impurity. It may 
be detected by the unpleasant odor remaining w^hen a few 
drops of the liquid are rubbed between the hands, and by the 
faint red tint which it imparts to a solution of nitrate of silver 
in the suspected liquid when exposed to sunshine. 


Aldehyde (Acetyl or Acetic Aldehyde). 

Formula, C2FT40 : Combining 'weight, 4 i. 

The term aldehyde is applied to a series of compounds 
which are derived from the alcohol series by the elimination 
of some of their hydrogen. Acet-aldehyde is the one with 
which photographers are principally concerned. It is a color- 
less volatile liquid having an odor like sweet spirits of nitre. 
It is formed by the oxidation of ordinary alcohol, which may 
be effected by the presence of atmospheric oxygen, or by nitric 
acid, etc. Aldehyde almost always appears in the nitrate of 
silver bath used in the wet collodion process. It may be 
removed by pouring the bath into an open dish and exposing 
it to sunlight for a few hours. This sunning the bath,” as it 
was called, had frequently to be resorted to in the old 
wet-plate ” days ; boiling has the same effect, but is not so 
safe a remedy. Acetic acid is also liable to contain traces of 
aldehyde. 

The presence of aldehyde in photographic solutions is 
injurious, as it is a powerful reducing or deoxidizing agent and 


CHEMICALS EMPLOYED IN I»HOTOGEAPHY. 


65 


causes metallic silver to be deposited as a bright mirror. In 
this way, however, it improves the tone of collodion transpa- 
rencies by completing the reduction of the silver forming the 
image. When aldehyde is oxidized it forms acetic acid 



CHAPTEH XL 


CHEMICALS EMPLOYED IN PHOTOGRAPHY 
(CONTINUED). 

Aluminum, ok Aluminium. 

Symbol, A1 : Atomic weight, 27. 

This metal (first isolated by Wohler in 1828) is prepared by 
heating the mineral known as cryolite with sodium ; also by 
the decomposition of compounds containing it in an electric 
furnace. Recent improvements in the manufacture have 
reduced the price of the metal from six dollars a pound to less 
than one dollar. Aluminum is a white, lustrous and very 
light metal ; being little more than one-quarter the weight of 
copper — bulk for bulk. It does not rust in air. Owing to its 
lightness, aluminum is coming largely into use for lens-mounts, 
and for the metal parts of cameras, tripod-heads, etc. 

Aluminium Ammonium Sulphate (Ammonia Alum). 
Formula Al^ (SOJ 3 , (XHJ^ SO^ + 24:11^0. 

When the ammonia liquor obtained in the manufacture of 
coal-gas is added to roasted coal-measure shale (which has 
been previously heated wdth sulphuric acid) ammonia alum is 
formed, which is then purified by crystallization. The appear- 
ance and properties of this salt are almost precisely similar to 
those of potash alum. Since the introduction of cheap potash 
salts from Stassfurt, in Germany, the manufacture of ammonia 
alum has almost ceased, potash alum taking its place. 

Aluminium Potassium Sulphate (Common Potash Alum). 

Formula, Al 2 (S 04 ) 3 , K 0 SO 4 + 24 IT 2 O : Combining weight, 

516 + 252=768. 

Common potash alum is a double salt formed by the com- 
bination of the sulphates of aluminium and potassium. It is 


CHEMICALS EMPLOYED IN PHOTOGKAPHY. 


67 


largely prepared by adding the latter compound to the roasted 
alum shales of the upper coal measures, which contain the 
former. 

Potash alum forms transparent regular octahedral crystals 
(double pyramids), which are soluble in ten parts of cold, or 
one-third their weight of boiling water ; insoluble in alcohol. 
The solution has an acid reaction and an astringent taste. By 
exposure to air the crystals turn white, owing to the absorption 
of ammonia and the formation of a basic sulphate. 

When heated they melt at 200 deg. F., in their water of 
crystallization, which then evaporates, leaving a white porous 
mass called burnt alum, which dissolves slowly in water. 

In photography alum is mainly employed to give firmness 
and insolubility to gelatine films when soaked in it. In com- 
bination with citric acid it also clears films which have been 
discolored by the pyro developer. When alum ” is spoken 
of, the common potash alum is always to be understood. 

Aluminium Nitrate. 

Formula, A12(N03 )q +16 II2O : Combining weight, 326. 

Prepared by dissolving aluminium hydrate in nitric acid, 
and evaporating the solution. It forms deliquescent needle- 
like crystals which are decomposed at a temperature of 302 
deg. F , leaving a residue of alumina. 

Aluminium nitrate is used as a mordant in calico printing. 

Aluminium Sulphate. 

Formula, Al3(S04))3 + I8II2O : Combining weight, 342 + 

324=666. 

Sulphate of alumina is produced commercially by decom- 
posing china clay with sulphuric acid. • It forms thin flat 
pearly plates, which dissolve in twice their weight of cold 
water. The pure salt for chemical purposes is prepared by 
adding aluminium hydrate to sulphuric acid. 

Amber. 

Amber is a fossil gum or resin which is found in the sandy 


68 


THE CHEMISTRY OF PHOTOGRAPHY. 


coast of North Germany, fringing the Baltic Sea. It is hardy 
brittle, yellow, and more or less transparent. In photography 
it is used (by dissolving the powdered amber in chloroform or 
benzole) to make" a varnish which can be applied cold to the 
surface of negatives. 

Ammonia. 

Formula, Nil 3 : Combining weight, IT- 

True ammonia is a light, colorless gas, which has a pungent 
odor, and is so soluble in water that a pint of water at the 
ordinary temperature will dissolve 730 pints of gaseous ammo- 
nia. It is this solution — ammonium hydrate, NH 4 HO — 
which is commonly called ‘^ammonia,” it is the ^Hiquor 
arnmonicB forV^ of druggists, and should have a specific 
gravity of .880. Ordinary liquid ammonia,” such as is 
used for pharmaceutical purposes, contains only ten per cent. 
of gaseous ammonia, and has a sjDecific gravity of .936. By 
heat, the ammonia gas can be driven out of the water, and 
even under ordinary circumstances the gas escapes so rapidly 
that the solution is perceptibly weakened by simply pouring it 
from one bottle to another. For this reason it is best to dilute 
the strong liquor ammonige, immediately after it is purchased, 
with an equal bulk of water, and to keep it in a well-stoppered 
bottle ; a corked bottle should never be emplo}^ed for this 
liquid. 

Ammonia gas is usually prepared by heating in a glass 
flask a mixture of quicklime and ammonium chloride. The 
ammonia gas which comes off will not burn unless it is first 
heated. The gas turhs moist red litmus to an intense blue, 
and makes yellow turmeric paper brown. For this reason, 
and from its propensity to escape from solution, ammonia is 
known as the ‘‘ volatile alkali.” Carbonate and chloride of 
ammonia are not unfrequently present, as impurities, in com- 
mercial ammonia. 

A trace of ammonia is always present in the air, and as some 
must be brought down by every shower, we see one way in 
which this substance — the essence of most manures — is natu- 
rally supplied to plants. Ammonia joins with the various 
acids to form ammoniacal salts, which greatly resemble the 


CHEMICALS EMPLOYED IN PHOTOGEAPHT. 


69 


corresponding compounds of potassium and sodium. Any 
ammoniacal salt can be easily recognized by the evolution of 
ammonia, wbicli occurs when the salt is warmed with a little 
slaked lime. All animal substances give off ammonia when 
they decay, or when they are heated. Formerly ammonia 
was mainly obtained by distilling the horns of deer in closed 
vessels, and hence its common name spirits of hartshorn,” 
or simply ‘‘hartshorn.” The whole of the ammonia and 
ammonia salts of commerce are now derived from the “ammo- 
nia liquor ” of gas-works. This liquor is neutralized with 
sulphuric or hydrochloric acid, and the resulting salts puritied 
by crystallization, or by sublimation. Ora current of steam 
is blown through the liquor. This carries with it the ammonia, 
and is passed through dilute sulphuric acid, when crystals of 
ammonia sulphate separate out. 

In photography ammonia is largely used to render the 
pyrogallic acid developer alkaline, the energy of its action 
being thereby greatly increased. 

Ammonium. 

Symbol, NH 4 : Combining weight, 18. 

Ammonium is one of those substances to which the name 
of comjpound radicle has been applied. It consists of the two 
elements, nitrogen and hydrogen ; yet it can be transferred 
from one compound to another just as if it were an element 
It has never been obtained by itself ; but its compounds 
behave in a similar manner to those of potassium. For these 
reasons chemists consider the group Nil 4 as a quasi-metal, 
and the name ammonium has been applied to it. 

Ammonium Bicheomate. 

Formula, (NIl 4 ) 2 Cr 2 0^ : Combining weight, 252. 

Prepared by adding chromium trioxide to ammonia. An- 
other method is to divide a solution of chromic acid into 
two parts; neutralize one part with ammonia, then add the 
other part and evaporate. Ammonium bichromate is a crys- 
talline substance, soluble in water. When a little of the 
solution is added to a solution of albumen, or gelatine, and 


70 


THE CHEMISTRY OF PHOTOGRAPHY. 


exposed to light it renders these substances insoluble in those 
liquids which would otherwise dissolve them. The cause of 
this appears to be that the bichromate suffers reduction by the 
action of the light, parting with some of its oxygen, which 
goes to the albumen, etc. The oxidized albumen, or gelatine, 
is insoluble in warm water and other solvents which readily 
dissolve the normal substance. Used in this manner the 
bichromate of ammonia renders great service in certain 
photographic processes. 

Ammonium Bromide. 

Formula, UH 4 Br : Combining weight, 9S. 

May be prepared by adding potassium bromide to a solution 
of ammonia, and evaporating, or by passing ammonia into 
hydrobromic acid. Its crystals are cubical and colorless ; very 
soluble in water, less soluble in alcohol and ether. This salt 
keeps well, but in contact with moist air it turns yellow, owing 
to the separation of bromine. When strongly heated it sub- 
limes without fusing. 

Ammonium bromide is largely used in photography. As 
an ingredient of the ordinary pyro developer it exercises a 
restraining action on the silver salts present in the film, thereby 
tending to the prevention of fog. 

Ammonium Carbonate. 

Formula, (NH 4 ) 2 C 03 : Combining weight, 99, 

Carbonate of ammonia (often called “ smelting salts,’’ and 
‘Ual volatile”) is made by heating a mixture of ammonium 
chloride and chalk. It is usually sold as a fibrous translucent 
solid, which smells distinctly of ammonia. It is insoluble in 
alcohol ; but soluble in three times its own weight of water. 
Ammonium carbonate may be used as the alkali in the pyro 
developer. It gives a pink tone to lantern slides and trans- 
parencies. 

Ammonium Chloride. 

Formula, NII 4 CI : Combining weight 53^. 

Prepared by adding hydrochloric acid to the ammoniacal 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


71 


liquor obtained from coal in the manufacture of coal-gas. It 
is purified by lieating until it sublimes, which it does without 
previous fusion. When prepared in this way on a large scale 
it forms tough, fibrous whitish lumps, which are soluble 
in three parts of cold or one part of boiling water ; insoluble in 
absolute alcohol. 

Ammonium chloride is largely used for “salting” paper 
which is to be subsequently sensitized with the silver nitrate 
bath. 

Ammonium Citrate. 

Formula, CgIIg(NA 4 ) 30 ^ + SllgO: Combining 

weight, 213 + 54 = 297. 

Prepared by neutralizing a solution of citric acid with am- 
monia. It is a powerful restrainer, and is of great service in the 
development of over-exposed negatives. As soon as all detail 
is out, the negative should be transferred to a three-per-cent 
solution of the citrate. After soaking for five minutes, de- 
velopment may be resumed ; when the negative will be found 
to intensify, without fog appearing. 

Ammonium Iodide. 

Formula, NH^I: Combining weight, 145, 

Prepared by adding ammonium sulphate to a hot saturated 
solution of potassium iodide. Alcohol is then added to pre- 
cipitate the potassium sulphate formed, which is then filtered 
otf, and the remaining solution evaporated, by which colorless 
cubes of ammonium iodide are obtained. 

This salt is a favorite in the collodion process, as it imparts 
to the collodion “ limpidity, sensitiveness, and adherency to the 
glass.” It is rather liable to decompose, especially when in 
contact with air, liberating iodine, which is known by its yel- 
low color. This tendency may be checked by the admixture 
of cadmium iodide. 

Ammonium Nitrate. 

Formula, (NII4) NO 3 : Combining weight, 80. 

This salt may be prepared by neutralizing solutions of 
ammonia or ammonium carbonate witli nitric acid. 


72 


THE CHEMISTRY OF PHOTOGRAPHY. 


On evaporation it crystallizes out in hexagonal prisms, 
which dissolve (producing great cold in so doing) in their own 
weight of water. 

Ammonium nitrate is decomposed by heat into nitrous 
oxide and water. 

Ammonium Sulphydrate. 

Formula, (NH^) HS : Combining weight, 51. 

Prepared as an aqueous solution by passing sulphuretted 
hydrogen through an aqueous solution of ammonia. It can 
also be obtained in the crystalline form by passing the same 
gas through an alcoholic solution of ammonia. Ammonium 
sulphydrate turns yellow when exposed to the air for any 
length of time. It precipitates many metals as sulphides, and 
is often used in this way to recover silver from solutions con- 
taining it. It is also useful as an intensifier. 

Antimony Sulphide. 

Formula, SbgSgi Combining weight, 340. 

Occurs native as a shining crystalline substance, having a 
leaden-gray color and a radiated structure. It may be pre- 
pared by heating together antimony and sulphur ; or by pass- 
ing a current of sulphuretted hydrogen through a solution of 
tartar emetic or any other soluble antimonious salt, when it 
appears as an orange-colored precipitate. 

Arsenic P)Romide. 

Formula, AsBrg-. Combining weight, 315. 

Prepared by dissolving powdered arsenic in a solution of 
bromine in carbon bisulphide, and evaporating. It forms 
colorless deliquescent crystals, which melt at about 70 deg. 
F., and are decomposed by water. 

Asphaltum. 

Asphal turn, bitumen, or bitumen of Judea, is a term which 
includes several substances occurring naturally in the earth. 
They are of a lirownish-black color, with a peculiar “ bitu- 
minous smell, and occur chiefly in volcanic regions, such as 


CHEMICALS EMPLOA^ED IN PIIOTOGRAPHAL 


73 


the Dead Sea, in Syria, the great Pitch Lake on the island of 
Trinidad, Cnba, Peru, etc. Chemically they afe sulphuretted 
hydro-carhous, and were prohalily formed hy the action of 
heated sulphur upon petroleum, or some similar body, under- 
ground. Analysis gives the average percentage composition 
as carbon 80, hydrogen 10 , sulphur 9, nitrogen and ash 1 . 

The term bitumen is usually employed for the softer or more 
huid kinds, while the name asphaltum, or asphalt, is applied 
to the hard varieties. Asphaltum is partially soluble in 
alcohol, ether or benzole ; very soluble in chloroform, carbon 
bisulphide, and turpentine. Asphalt is the foundation of most 
of the black varnishes now in use. In photogi’aphy this sub- 
stance has a special interest, as it was upon metal plates 
covered with a thin layer of asphalt that Niepce obtained the 
first permanent photographs about the year 182d. 

Asphalt is affected by light with the result that it is rendered 
insoluble in its usual menstrua. For this reason it has long 
been employed in photo-lithography, in which its indifference 
to acids is also of value. Nicephore Isiepce first employed 
asphalt in this way, in 1824, in his photographic process called 
lieliograijliy. 

Azaline. 

This is the commercial name for a mixture made by dissolv- 
ing 30 grains of quinoline-red and 3 grains of quinoline-blue or 
cyanin in 40 ounces of alcohol. It is used to produce iso- 
chromatic effects ; causing plates which are bathed in the 
solution to become more sensitive to the yellow and red rays. 

Barium. 

Symbol, Ba.: Combining weight, 1 37. 

Metallic barium was not obtained until the year 1808, when 
Davy isolated it by the electrolysis of baric chloride. It is a 
yellow metal, as easily oxidized as sodium, decomposing water 
at the ordinary temperature. 

Barium Chloride. 

Formula, BaCl^ + 2 H 3 O: Combining weight, 208 + 36=244. 

F'repared by dissolving barium carbonate in hydrochloric 


74 


THE CHEMISTRY OF PHOTOGRAPHY. 


acid. The colorless crystals of barium chloride usually met 
with are fairly^ soluble in water, and the solution is used as a 
test for sulphuric acid or any soluble sulphate. The presence 
of these substances is indicated by a heavy white precipitate of 
barium sulphate, insoluble in all acids except hot, strong sul- 
phuric acid. 

Barium Hydrate. 

Formula, BaHgOg + 8 H 3 O : Combining weight, 171 + 144=315. 

Made by dissolving barium nitrate in a hot solution of caus- 
tic soda. It forms white crystals which are soluble in twenty 
times their weight of water; insoluble in alcohol. Also 
known as baryta,” and as ‘‘barium hydroxide.” 

The solution absorbs carbonic acid from the air and soon 
becomes milky. Dr. A. II. Elliott states that barium hydrate 
acts as an accelerator with hydroqiiinone ; no bromide must 
be used. 

Barium Nitrate. 

Formula, Ba(N 03 ) 2 : Combining weight, 261. 

Barium nitrate is commonly called “nitrate of baryta.” It 
is prepared by dissolving barium carbonate or sulphide in dilute 
nitric acid. Its crystals are soluble in twelve parts of cold, or 
three of boiling, water; insoluble in nitric acid or in alcohol. 

The addition of a little barium nitrate to the silver nitrate 
bath used in the wet collodion process prevents the formation 
of “pin-holes.” 

Benzole (or Benzene). 

Formula, CgH^: Combining weight, 78. 

Commercially, benzole is obtained from coal-tar oil, of which 
it is the most volatile constituent. It is a white solid which 
melts at 42 deg. F. to a clear, limpid liquid having a peculiar 
and rather pleasant smell ; it boils at 177 deg. F., and the vapor 
burns with a bright but smoky flame. Benzole is not soluble 
in water but dissolves freely in alcohol, ether, and oil of tur- 
pentine. It is an excellent solvent for caoutchouc and gutta- 
percha, and dissolves fats and oils with such facility that it is 
in general use for removing grease spots. 


CHEMICALS EMPLOYED IN PHOTOGKAPHY. 


75 


Bromine. 

Symbol, Br : Combining weiglit, 80. 

The element bromine was first obtained by Balard, in 1826, 
from the salts left by the evaporation of sea-water. It is a 
dark-red, heavy liquid, which becomes a black solid when its 
temperature is lowered to eight degrees below zero (F.), and 
which boils at 145 deg. F. There are only two elements which 
are liquid at ordinary temperatures ; bromine is one and mer- 
cury the other. Bromine has a strong irritating smell, and is 
very poisonous. It is prepared by heating potassium bromide 
with sulphuric acid and black oxide of manganese. 

2 KBr ■+ + MnO^ = Br^ + K^SO^ 

Potassium bromide. Sulphuric acid. Manganic oxide. Bromine. Potassium sulphate. 

+ MnSO^ -I- 2H2O 

Manganese sulphate. Water. 


it 



CHAPTER XIL 


CHEMICALS EMPLOYED IN PHOTOGRAPHY 
(CONTINUED). 

Cadmium. 

Symbol, Cd : Combining weight, 112. 

This metal was discovered in 1817. It is usually found 
combined with zinc in the various ores of the latter metal, and 
for this reason zinc is a common impurity in the commercial 
salts of cadmium. Cadmium is a white lustrous metal, resemb- 
ling tin. It is attacked by the stronger acids. Zinc precipi- 
tates metallic cadmium from any solution containing it. In 
photography powdered cadmium is sometimes used to remove 
free iodine from collodion. 

Cadmium Jeomide. 

Formula, CdBrg + 4 H 3 O : Combining weight, 

272 + 72=314. 

Prepared by digesting powdered metallic cadmium with 
bromine and water. By evaporating the solution, needle- 
shajDed crystals of CdBrg, combined with four equivalents of 
water, are obtained. By heating carefully in a porcelain cru- 
cible the water of crystallization may be removed. 

Owing to the stability of this salt, and its solubility in col- 
lodion, alcohol, and ether, it has been much used as a source of 
the bromine which is required for the production of silver bro- 
mide — the sensitive compound now so universally employed in 
photography. 

Cadmium Iodide. 

Formula, Cdig : Combining weight, 366. 

Prepared by digesting the powdered metal with iodine and 
water. By evaporating the solution cadmium iodide is ob- 


CHEMICALS EMPLOYED IN PHOTOGEAPHY. 


77 


tained in fiat, pearly crystals. It is soluble in water, and is 
also one of tlie few iodides which are soluble in alcohol. For 
the latter reason it is largely used in photography for the pur- 
pose of iodizing the collodion used in the wet process. 

Calcium Bromide. 

Formula, CaBrg : Combining weight, 200. 

Obtained in silky needles when hydrobromic acid is passed 
into an aqueous solution of calcium hydrate (slaked lime), and 
the liquid evaporated. Its properties are similar to those of 
calcium chloride. 

Calcium Carbonate. 

Formula, CaCOg . Combining weight, 100. 

Carbonate of lime occurs plentifully as limestone, chalk, 
marble, and Iceland spar ; the last two forms being nearly 
pure. Insoluble in alcohol, and in pure water; slightly soluble 
in water which contains carbonic acid gas. Whiting,” or 
“ whitening,” is powdered chalk ; it is used to neutralize acidity 
in the ordinary gold toning baths. 

Calcium Chloride. 

Formula, CaCl^ +6H2O : Combining weight, 

111 + 108 = 219 . 

Prepared by dissolving marble (calcium carbonate) in hy- 
drochloric acid ; also obtained as a bye-product in the manu- 
facture of ammonia and potassium chlorate. 

Calcium chloride forms large transparent crystals, which are 
extremely soluble in water, producing great cold, and deliquesce 
when exposed to the air. It is also freely soluble in water. 
By a strong heat the water of crystallization can be driven off, 
and the pure anhydrous salt remains as a white or colorless mass. 
In this state it greedily absorbs water, and is much used for 
drying gases and liquids. For the latter purpose it is best to 
place lumps of the anhydrous fused salt in the liquid. Of 
course, only liquids in which calcium chloride is not soluble 
can be dried in this way. Gases are usually dried by passing 


78 


THE CHEMISTRY OF PHOTOGRAPHY. 


them through tubes full of small lumps of the white salt. The 
air in the box used for drying gelatine plates can be dried by 
keeping a metal box filled with calcium chloride at the bottom 
of the box. Calcium chloride (wrapped in tissue-paper and 
cotton-wool) is also used to dry the air in the paper or metal 
tubes in which platinotype paper is usually kept. 

Calcium Hydrate. 

Formula, CaHgOg : Combinmg weight, 74. 

Made by pouring water upon quicklime. Is also known as 
‘‘slaked lime,” and as “calcium hydroxide.” Soluble in 700 
times its own weight cf water, the solution being known as 
“ lime-water.” Is sometimes used in photography in the gold 
toning bath. In the hydroquinone developer lime-water (com- 
bined with sulphite of soda and sugar) acts as an energetic 
accelerator. 

Calcium Iodide. 

Formula, Caig : Combining weight, 294. 

Prepared by dissolving calcium carbonate in hydriodic acid. 
In its properties it resembles calcium chloride. When heated 
in air it parts with the whole of its iodine, and fcrms calcium 
oxide. 

Calciuxi Oxide (Quicklime). 

Formula, CaO : * Combining weight, 56. 

Ordinary “ quicklime ” is prepared by heating carbonate of 
lime (limestone) in kilns, the heat driving olf the carbonic acid 
gas: 


CaCOg 

= CaO -f 

CO., 

Carbonate of lime. 

Quicklime, 

Carbonic acid gas, 


Quicklime rapidly absorbs moisture from the air, and crumbles 
away ; hence it should be kept in well-stoppered bottles. 
Cylinders of hard quicklime heated by an oxy -hydrogen 
flame are used as a source of light in the oxy-hydrogen 
lantern. 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


79 


Camphor. 

Formula, gO : Combming weight, 152. 

Camphor is obtained bv steaming the wood of the camjplior 
laurel^ a tree which grows in China and Japan, It is a white 
or colorless crystalline solid of penetrating odor, only slightly 
soluble in water, but soluble in alcohol and ether. Camphor 
is also soluble in turpentine, and the solution, mixed with a 
little emery, is very useful in grinding glass stoppers into the 
necks of bottles so as to secure a perfect lit, or for grinding 
glass for any purpose. Camphor is very tough, but can readily 
be pounded up when mixed with a little of any of the liquids 
which dissolve it. It is used as a preservative, keeping oh the 
attacks of insects and bacteria, and so preventing solutions of 
gelatine, albumen, etc., from becoming mouldy. 

The addition of a little camphor (a piece about the size of a 
nut to each pint) to the oil used in magic lanterns is found to 
increase the brilliancy and whiteness of the light. 

Canada Balsam. 

Canada balsam is a resinous substance containing much 
essential oil, which causes it to be soft and viscous. It exudes 
from incisions made in the stem of a species of pine-tree 
{Phius halsamcB)^ which grows abundantly in Canada. From 
its liquid and colorless sticky nature it is much used by opti- 
cians for cementing together the components of achromatic 
lenses. Some samples, after long exposure to light, turn yel- 
low, while others crack and show the ‘‘ colors of thin plates,” 
causing a fear that the Tens is damaged. When this is the 
case the lens should be removed from its brass mount and 
soaked in warm turpentine, which will dissolve the cement. 
The ordinary Canada balsam of commerce is of a yellowish hue, 
but it can be decolorized by exposing the yellow balsam in 
clear white glass bottles to sunlight. 

Canada balsam dissolved in benzole renders paper translucent. 

Caoutchouc. 

Caoutchouc, more familiarly known as India-rubber, is the 
solidified juice which exudes from certain tropical plants. 


80 ^ THE CHEMISTRY OF PHOTOGRAPHY. 

When protected from air and light (as by being kept in "water 
in a dark place) it undergoes no change, but under ordinary 
conditions it absorbs oxygen from the air, and becomes rotten 
and inelastic in the course of a few months. 

Freshly cut edges of caoutchouc adhere firmly when brought 
into contact, and it is invaluable in the laboratory for the con- 
struction of tubing, etc. Washed ether, chloroform, carbon 
bisulphide, coal-naphtha, and rectified oil of turpentine are all 
able to dissolve caoutchouc. It is insoluble in alcohol. When 
caoutchouc is heated with 2 or 3 per cent, of sulphur, the 
compound known as vulcanized India-rubber is formed. If 
the percentage of sulphur be increased to 12 or 15, the heated 
mixture becomes hard, black, and horny, and is known as 
ebonite or vulcanite. 

Gutta-percha is the hardened juice of a tree which grows in 
Singapore, Borneo, etc. Its properties are similar to those of 
caoutchouc. 

Carbolic Acid (Phenol). 

Formula, CgHgO: Combining weight, 94, 

Coal-tar is the principal source of carbolic acid. When 
purified it crystallizes in colorless needles, which melt at 102 
deg. F. It is soluble in water, and still more soluble in alcohol, 
ether, and acetic acid. Although called an acid, it does not 
redden litmus paper. Of late years carbolic acid has been 
largely used as a disinfectant, and as a preventer of putrefac- 
tion and fermentation. These valuable qualities appear to be 
due to its power of coagulating albu#ien. When a few drops 
of an aqueous solution of carbolic acid are added to albumen, 
gum, etc., decay or mould will be prevented. Carbolic soap 
contains from five to twenty per cent, of the acid, and is most 
useful, not only for general purposes, but in special cases where 
a disinfectant is required. 

Carbon. 

Symbol, C : Combining weight, 12. 

Carbon is found free in nature as the mineral graphite 
(commonly called black-lead or plumbago), and crystallized as 
the diamond. Coal usually contains from three-quarters to 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


81 


nine tenths its weight of carbon. All organic compounds con- 
tain carbon, and they give evidence of this by hlachening when 
heated. In this way we often notice the presence of this ele- 
ment in bread, meat, etc. 

By heating coal, wood, or bones in iron retorts, the gases 
these substances contain are driven off, and the forms of car- 
bon known as coke, charcoal, and bone black are left behind. 
Lamp-black is a finely divided form of carbon deposited from 
burning oil or tallow ; and in the same way we get gas-black 
by holding some cold incombustible body in a gas flame. 

Carbon has never been melted or dissolved. Amorphous 
carbon or charcoal has a remarkable power of absorbing and 
condensing gases. In this way it destroys bad smells, and pre- 
vents putrefaction. It also retains the coloring matter of 
liquids passed through it, and is used for this purpose in the 
purification of raw sugar, and in filters, etc. 

Carbonic Acid G-as. 

Formula, COg : Combining weight, 44. 

Carbonic acid gas is chemically known as carhonic anhy- 
dride^ because, when added to water, it produces a feeble acid 
— the true carbonic acid, II3CO3. But this acid cannot be 
obtained pure, and its aqueous solution is very unstable. 
Carbonic acid gas is usually prepared by the action of dilute 
hydrochloric acid on marble (calcium carbonate) ; but it is re- 
leased whenever one of the carbonates is treated with a stronger 
acid, and is formed whenever carbon, or any substance con- 
taining carbon, is burnt in the air. 

Carbonic acid gas is colorless ; it will not burn, nor will it 
support combustion ; it is so heavy that it can readily be 
poured, like a liquid, from one vessel to another. Animals 
soon die when placed in an atmosphere of this gas, and many 
human lives have been lost owing to its accumulation at the 
bottom of old wells, brewers’ vats, etc. By cold and pressure 
combined, carbonic acid gas can be reduced to a colorless 
liquid whose evaporation can be made to produce a most in- 
tense cold — .106 deg. F. 


82 


THE CHEMISTKY OF PHOTOGRAPHY. 


Commercially, carbonic acid gas is largely used in the manu- 
facture of effervescent drinks, such as soda-water, ginger-beer, 
etc., the gas being forced into the liquid by pressure. 

The sparkling appearance of spring-water, champagne, and 
most aerated waters is due to the presence of carbonic acid gas. 

Castor Oil. 

This is a viscid oil obtained from the seeds of the castor 
oil plant,” jRicinus communis. It slowly hardens by long ex- 
posure to the air, but does not solidify even at 0 deg. F. It is 
soluble in alcohol. When a small quantity of castor oil is 
mixed with collodion it toughens the film so that it can be 
more readily transferred from the glass plate to some other 
support. It also imparts a toughness to varnishes. 

Chlorine. 

Symbol, Cl ; Combining weight, 35^. 

Chlorine was discovered by Scheele, in 1774. It is never 
found free in nature, but occurs plentifully combined with 
sodium (as common salt, NaCl), and with many other metals, 
forming binary compounds called chlorides. 

It is a greenish-yellow heavy gas, possessing a powerful and 
disagreeable smell (something like that of sea-weed). It is 
very dangerous to inhale chlorine ; hence it should always be 
prepared in the open air or where there is a free draught. 

For this purpose we may mix one ounce of salt with one 
ounce of black oxide of manganese in a glass retort, and then 
add two ounces of sulphuric acid, previously dilated with an 
equal quantity of water. When a very gentle heat is applied, 
chlorine gas will come off in abundance. It should be washed 
by passing it through water. 

By submitting chlorine to a pressure of about seventy-five 
pounds per square inch, it is converted into a heavy yellow 
liquid. 

Chlorine is very soluble in water, and the solution — known 
as chlorine-water ” — is used for many purposes instead of 
the pure gas. Its powers of combination with other elements 
are very marked. A mixture of chlorine with hydrogen ex- 


% 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


83 


plodes when exposed to sunlight or to the light of burning 
magnesium, the two elements combining to form hydrochloric 
acid gas. 

When metals in the state of a fine powder are dropped into 
chlorine gas, they take fire spontaneously, and the chlorides of 
the metals are formed. 

Chlorine l)leaches all animal and vegetable colors, and it is 
largely used for this purpose in the manufacture of paper, of 
cotton, and of linen. If all traces of chlorine are not removed 
after bleaching is effected, the substance will rapidly rot. 
Hyposulphite of soda — the photographer's bane — is frequently 
used to effect this complete removal of the last traces of chlo- 
rine, and is hence termed an anti-chlor.” 

Chlorinetted Lime (Calcium Ch loro-hypochlorite or 
Bleaching Powder). 

Formula, Ca(OCl)Cl : Combining weight, 12T. 

This substance is commonly known as ‘^chloride of lime” 
and as ‘^bleaching powder.” Chemists are not fully agreed 
respecting its chemical nature, some regarding it as a mixture 
of calcium chloride with calcium hypochlorite, while others 
consider it to be a true chemical compound — calcium chloro- 
hypochlorite. 

Bleaching powder is made on a very large scale in the alkali 
works of South Lancashire. The fioors of largre chambers are 
covered with dry slaked lime, and the chambers are then filled 
with chlorine gas, which combines witli the lime. Commercial 
bleaching powder contains from twenty-five to thirty-five per 
cent, of available chlorine. It is a white powdei*, which has a 
faint smell of hypochlorous acid, and attracts moisture from 
the air. For bleaching purposes the articles are first dipped in 
a clear dilute solution of the bleaching powder, and then 
placed in very dilute hydrochloric acid. In this way chlo- 
rine is liberated, which combines with the coloring matters to 
form colorless compounds. 

Under the name of chloride of lime,” bleaching powder is 
largely used as a disinfectant. In photography it is used as 
an ingredient of a toning bath which gives black tones. 


84 


THE CHE:\riSTKY OF PHOTOGRAPHY. 


Chloroform. 

Formula, CHCI3 : Combining weight, 

Chloroform can be prepared in several ways, as by distiiling 
bleaching powder with very dilute alcohol, or by the action of 
chlorine on marsh gas. It is a colorless, heavy, volatile liquid, 
having a strong and rather agreeable smell. When inhaled it 
produces perfect, though temporary, insensibility to pain. It 
is a good solvent for sulphur, phosphorus, and iodine, and for 
most fatty and resinous bodies, especially caoutchouc. It has 
no action on collodion, and does not mix with water ; it dis- 
solves readily in alcohol. 

Chlorophyll. 

The name given to the green coloring matter of plants. It 
is prepared by treating chopped leaves (young myrtle leaves 
answer well) with warm alcohol for ten minutes, and then 
filtering. It should be kept in an opaque bottle, with a little 
powdered zinc. Chlorophyll is insoluble in water ; soluble in 
alcohol and in ether. It has been successfully used by Ives 
and others to render gelatine plates more sensitive to the red 
and yellow rays. 

Chromium Potassium Sulphate (Chrome Alum). 

Formula, Cr3(S04)3, K3SO4 +24H2O : Combining 

weight, 566 + 432=998. 

Prepared by passing sulphurous acid gas through a mixture 
of potassium bichromate and sulphuric acid. Also obtained 
as a bye-product in the manufacture of alizarine. 

Chrome alum forms octahedral crystals, dark-red — almost 
black — in color, soluble in seven parts of water. 

Chrome alum is employed in tanning. In photography it is 
used to toughen and render insoluble the films of gelatine used 
in the manufacture of dry-plates. 

Citric Acid. 

Formula, C ^ II g O .^ + II 3 O : Combining weight, 

" 192 + 18=210. 

Citric acid is principally prepared from the juice of lemons, 
by the addition first of piowdered chalk and then of sulphuric 


CHEMICALS EMPLOYED IN PHOTOGKAPHY. 


85 


acid, the chalk forming calcium citrate, which is decomposed 
by the acid. Citric acid forms transparent crystals, which are 
very soluble in water and in alcohol. Being a tri-basic acid, it 
forms three series of citrates, of which those of the alkalies are 
soluble in water. When added to the pyro developer, citric 
acid checks strongly the reduction of the silver salt, so that it is 
frequently used as a retarder, being especially useful in hot 
weather, or when the exposure has been much too long. 

Collodion. 

In 1847, Maynard, in America, discovered that a certain 
form of pyroxyline was soluble in a mixture of alcohol and 
ether, and that as these solvents evaporated the pyroxyline 
was left behind as a delicate transparent skin or film. To the 
substance so obtained the name of collodion was given, and it 
was found to be of service in surgery to form a covering to raw 
places on the skin to keep away the air. 

In 1850, Scott Archer applied the new material to photo- 
graphic purposes, using it to coat glass plates, and to receive and 
hold the sensitive salts which wei’e to be affected by light. 
From 1850 to 1878 the ‘^collodion pro3ess” was almost univer- 
sally employed by photographers, but the advent of gelatine 
dry-plates in the latter year has since caused it to hold a sec- 
ondary position. 

For general work a good collodion may be made by taking 
half a pint of alcohol (sp. grav., .820) and the same quantity of 
ether (sp. grav., .725), and dissolving in the mixture 115 grains 
of pyroxyline. In cold weather half an ounce less alcohol and 
half an ounce more ether may be used with advantage. 

Photographers almost always purchase collodion ready-made, 
since the great manufacturing firms who have made its prepa- 
ration a specialty are able to produce a better article at a less 
cost than any individual could hope to do. 

Copper Bromides. 

Copper combines with bromine in two proportions to form 
cuprous bromide (Cu^Brg) and cupric bromide (CuBr^) ; the 
combining weights are 287 and 223, respectively. 


86 


THE CHEMISTRY OF PHOTOGRAPHY. 


Cuprous bromide is a brown crystalline substance, which be- 
comes blue when exposed to sunlight. It can be prepared by 
heating copper tilings in contact with bromine. 

Cupric bromide is formed as dark-colored deliquescent crys- 
tals when cupric oxide is dissolved in hydrobromic acid and 
the solution evaporated m vacuo. 

Copper Nitrate. 

Formula, Cu(]SrO 3 ) 3 +H 3 O : ^ Combining weight, 

187+54=241. 

Copper nitrate is produced by the action of nitric acid on 
metallic copper, or on cupric oxide. It forms blue prismatic 
crystals, which are very soluble in water and in alcohol. 
Copper nitrate readily parts with oxygen, and is used as an 
oxidizing agent in dyeing and in calico printing. It imparts 
a green color to the flame of a spirit-lamp or Bunsen burner. 

Copper Sulphate. 

Formula, CUSO 4 + 5 H 3 O : Combining weight,, 

159 + 90=249. 

Copper sulphate, cupric sulphate, or hlue vitriol.^ is obtained 
in large blue crystals by dissolving copper oxide in dilute sul- 
phuric acid and evaporating the solution. It often contains 
ferrous sulphate as an impurity. 

Cyanin. 

Formula, CgglIggNgl: Combining weight, 526. 

Also known as chinolin blue, or quinolin. Sold as a coarse 
dark-green povv^der of metallic lustre, which is slightly soluble 
in water, more so in alcohol. Used to increase the sensitive- 
ness of gelatine dry-plates to the red rays. 

Cyanogen. 

Formula, (CN) 3 , or Cy 3 : Combining weight, 52. 

The important organic compound called cya,nogen was dis- 
covered by Gay Lussac in 1814. Cyanogen gas can be ob- 
tained by strongly heating dry mercuric cyanide in a glass 


CHEMICALS EMPLOYED IN PHOTOGKAPHY. 


87 


tube. It is transparent and colorless, and burns with a beauti- 
ful rose edged, purple flame. Cyanogen is very soluble in 
water and in alcohol. Cyanogen is important as being the 
first known of the “ compound radicals — compounds which 
can be transferred bodily from one chemical compound to an- 
other, just like elements. Its “ compound atom ” (CN) forms 
part of many organic substances. Cyanogen may be readily 
liquefied by heating mercuric cyanide in a bent tube, sealed 
at both ends. 

Dextrine. 

Also known as British Gum.” Made by moistening starch 
with dilute nitric acid, and then drying and heating. Sold as 
a brown powder which dissolves in hot water and is then used 
for mounting prints. Dextrine is the adhesive generally used 
to coat the backs of postage-stamps. 

Eikonogen. 

In 1881 Professor Raphael Meldola prepared a new sub- 
stance, to which he gave the name of amido-beta-naphthol- 
sulphuric acid.* Its chemical formula is C^ oHgNHgjOH, 
SO 3 II, and Meldola obtained it by the reduction of nitroso- 
beta-naphthol-sulphuric acid. It was soon afterwards obtained 
more cheaply by other processes by Dr. Witt, in Germany ; 
and is now manufactured by the Actiengesellschaft fiir Ani- 
linfabrikation in Berlin. 

Eikonogen is the sodium salt of this acid ; and its chemical 
name is therefore ^ sodium-amido-beta-naphthol-sulphonate. 
Its formula is C^ oH 5 NIl 2 ,OIsra,S 03 lI. Eikonogen is sold 
in yellowish-white crystals which are moderately soluble in 
water; insoluble in alcohol. Like pyro, hydroquinone, etc., 
eikonogen is able to reduce silver salts, and it forms a valuable 
developing agent. It is not poisonous. 

Erythrosine. 

Formula, CgIl 4 [COCgHl 2 (OI^a)o ]3 : Combining weight, 830. 

This is a reddish-brown powder obtained by the action of 


* Jourtial of the Chemical Society^ vol, xxxix., p. 47. 


88 


THE CHEMISTRY OF PHOTOGRAPHY. 


iodine upon fluorescein. It is also known as eosin blue- 
shade,” and as erythrosin B.” It is very soluble in water. 
Erytlirosine is the substance commonly employed to render 
gelatine emulsion or dry-plates sensitive to yellow and green 
light; and plates so prepared are known as “isochromatic,” 

orthochromatic,” or color-sensitive.” Its use (in conjunction 
with ammonia) for this purpose was patented by Attout- 
Tailfer in 1882. 

Ether (Sulphuric Ether). 

Formula, C 4 II 10 O: Combi nfng weight, 74. 

Ether is prepared on a large scale by distilling alcohol with 
sulphuric acid. It is a colorless, very mobile liquid which has 
a speciflc gravity of from .720 to .736, and a boiling point of 
94 deg. F. Ether is a transparent and light liquid having a 
fragrant and exhilarating smell. It mixes readily with alcohol, 
but scarcely at all with water. It dissolves fats and resins ; also 
bromine and iodine, and most metallic bromides and iodides. 

The boiling point of ether is so low, and it vaporizes so 
readily, that it is dangerous to bring a light near to an unstop- 
pered bottle containing it. Pore ether becomes acid by 
exposure to light, so that it should always be kej)t in a dark 
and cool place. This acid condition may be detected by the 
yellow color such ether produces when shaken up with an 
aqueous solution of iodide of potassium. Ether is sometimes 
adulterated with water, but the latter may be tested for by 
mixing a little of the ether with spirits of turpentine. If any 
w^ater is present a turbidity is produced. Methyl, or wood 
spirit, is sometimes added, but this may be known by the 
smell, and its discharging in a few hours the color produced 
by adding one drop of tincture of iodine to an ounce of the 
ether. One of the best signs, however, of pure sulphuric 
ether is its low speciflc gravity. 

Ether, Methylated. 

Just as pure ether is made by distilling a mixture of alcohol 
and sulphuric acid, so methylated ether is made by dis- 
tilling methylated spirit with the same acid. 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


89 


As methylated spirit is much cheaper than pure alcoliol, so 
the methylated ether produced is cheaper tlian pure ether. 
Since it answers almost equally as well as pure ether in the 
manufacture of collodion, the methylated ether is now univer- 
sally used hy large preparers of that article. Its odor is, hoW“ 
ever, stronger and more disagreeable than that of pure ether, 
and the nitrate of silver hath is more easily disorganized by it. 

Ether, Methylic. 

r ormula, C g H g O : Combining weight, 46 . 

Methyl ether, or methylic ether, is also known as methyl 
oxide. Although it is composed of the same elements, in the 
same proportions, as alcohol, yet its properties are very differ- 
ent, a fact which chemists explain by believing that the atoms 
which form its molecule are differently arranged. Methylic 
ether is prepared by distilling one part of methyl alcohol (or 
pyroxylic spirit, CII^O), with four parts of oil of vitriol, and 
purifying the distillate with slaked lime. 

It is a colorless gas, very soluble in water, and still more so 
in alcohol or ether. It burns with a pale flame. 

Ferrous Oxalate. 

Formula, FeC2 04 : Combining weight, 142 , 

Oxalate of iron is a yellow powder, which is formed as a 
precipitate when a solution of oxalic acid (one ounce to 16 
ounces of water) is added to an equal bulk of a solution of 
ferrous sulphate (2^ ounces to 16 ounces of water to which a 
few drops of sulphuric acid have been added). Allow to stand 
all night; then decant off the clear liquid and wash and dry 
the precipitate. Ferrous oxalate is insoluble in water, but 
readily dissolves in a solution of potassium oxalate, forming a rich 
red solution of potassium ferrous oxalate, K2Fe(C 204)3. In 
this state it forms a clear and vigorous developer, which is 
especially used for bromide paper and for transparencies. 
This developer is geiierally made by adding a saturated solu- 
tion of ferrous sulphate to a saturated solution of potassium 
oxalate (1 part of the former to 4 parts of the latter) but when 


90 


THE CHEMISTRY OF PHOTOORAPHY. 


produced in this way it must not be forgotten that sulphate of 
potassium is also formed, and this acts as a restrainer. 

F LUORINE. 

Symbol F : Combining weight, 19. 

This element, the most chemically active of the four 
halogens, is most frequently met with in combination with 
the metal calcium, as beautiful cubical crystals of calcium 
fluoride, CaFg (commonly called fluor-spar). 

Fluorine has a remarkable chemical affinity for all the other 
elements except oxygen. It was not obtained in the separate 
or free state till 1887, when M. Henri Moissan succeeded in 
obtaining it by passing a current of electricity through potas- 
sium fluoride dissolved in anhydrous hydrofluoric acid. 
Fluorine is a colorless gas, having a penetrating and disagree- 
able odor, and an irritating effect when inhaled. It combines 
instantly with almost all substances, even such refractory 
bodies as silicon, boron, etc., igniting spontaneously when 
brought into contact with it. 

Formic Acid. 

Formula, CHgOg : Combining weight, 46. 

This acid occurs in the bodies of red ants, in the hairs of 
certain species of caterpillars, and in stinging nettles. 

It is usually prepared by distilling a mixture of oxalic acid 
and glycerine. 

Formic acid is a clear, colorless, inflammable, corrosive 
liquid, which acts as a powerful reducing agent. It has been 
strongly recommended as a preservative for solutions of pyro- 
gallol. 

When heated with silver nitrate, carbonic acid gas is 
evolved and metallic silver deposited ; similarly mercuric chlo- 
ride is rednced to mercurous chloride or calomel. This reduc- 
ing action seems to distinguish formic acid from acetic acid 
and its homologues. 

Freezing Mixtures. 

By a mixture of certain chemicals, or other substances, it is 
possible to temporarily produce a degree of cold far below the 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


91 


freezing-point of water. It is advantageous to place the mix- 
ture ill some thick vessel, well surrounded hj flannel or some 
other non-conducting substance, to prevent the access of heat. 
The substance to be cooled should be placed in a thin vessel, 
which should be inserted in the middle of the freezing 
mixture. Thus the street vendors of “ice-cream” put the 
dainty in a tin, which stands in a freezing mixture contained 
in a bucket well wrapped up in flannel. 

Some of the best-known freezing mixtures are given below, 
together with the degree of cold each is capable of producing. 


Freezing Mixtures Without Ice. 


Mixtures. 

Thermometer falls. 

Degree 
of cold 
pro- 
duced. 

Ammonium nitrate, 1 part 

Water, 1 part 

i 

From 50 deg. to 4 deg. 

46 deg 

Ammonium chloride, 5 parts ) 

Potassium nitrate, 5 parts > 

Water, 1(5 parts ) 


From 50 deg. to 10 deg. 

40 deg 

Sodium sulphate (Glauber’s salt), 3 parts. ( 
Dilute nitric acid, 2 parts i 

From 50 deg. to -3 deg. 

53 deg 

Sodium sulphate, 8 parts 

Hydrochloric acid, 5 parts 

} 

f 

From 50 deg. to 0 deg. 

50 deg 

Sodium sulphate, C parts 

Ammonium nitrate, 5 parts * 

Dilute nitric acid, 4 parts 

i 

From 50 deg. to -14 deg. 

64 deg 


Freezing Mixtures With Ice. 


Mixtures. 

i 

Thermometer falls. 

Degree 
of cold 
pro- 
duced. 

Snow (or powdered ice), 2 parts ( 

Common salt, 1 part ( 

From any temperature 
to 5 deg. below zero. 


Snow, 8 parts ) 

Hydrochloric acid, 5 parts ) 

From 32 deg. to 27 deg. 

below zero 





Snow, 4 parts } 

Calcium chloride, 5 parts ( 

From 32 deg. to 40 deg. 
below zero 

72 deg 


All the temperatures given above are according to 
Tahrenheit’s scale. By the evaporation of a mixture of solid 


92 


THE CHEMISTRY OF PHOTOGRA.PHY. 


carbonic acid gas and sulphuric ether, a temperature of no less 
than 198 deg. F. below the freezing point of water can be 
produced. Several machines for the production of artificial 
ice have been invented. Of these perhaps Carre’s is the best 
known. In it the cold is produced by the evaporation of pure 
ammonia, which has previously been liquefied by pressure. 

The production of cold by freezing mixtures depends on 
the fact that to change bodies from the solid to the liquid, or 
from the liquid to the gaseous state, heat is required. When 
substances are dissolved, or vaporized, without the application 
of external heat — as when a salt is dissolved in water — the 
heat necessary to bring about the change of state is abstracted 
from the neighboring objects. 

Gallic Acid. 

Formula, C^HgOgi Combining weight, 170. 

Gallic acid is prepared from tannic acid by exposing to the 
air for several months moist powdered nutgalls (which contain 
nearly half their weight of tannic acid). The dark, mouldy 
mass so produced is first pressed, and then boiled with water, 
from which, on cooling, feathery colorless crystals of gallic 
acid are deposited. It is soluble in one hundred parts of cold 
or three of boiling water, has an acid, astringent taste, and 
decomposes when kept in solution. 

When heated to 400 deg. F. gallic acid gives off carbonic acid 
gas, and forms pyrogallic acid. Gallic acid difiers from tannic 
acid in not precipitating gelatine. With ferric salts it forms a 
black precipitate which disappears when heated. Gallic acid 
slowly reduces salts of gold and silver to the metallic state, 
and it is to this property that it owes its introduction into 
photography. 

Gallic acid was used as a developer by the Eev. J. B. Beade, 
it is said, as early as 1837, and certainly by Fox Talbot, in his 
calotype process, in 1840. It can also be employed in combi- 
nation with silver nitrate as an intensifier. 

Gelatine. 

Wlien a l)one is left for a day or two in a weak solution of 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


93 


hydrochloric acid, the mineral part is dissolved away, and a 
soft mass remains composed of a substance known ossein. 
An analysis of ossein shows it to be composed in one hundred 
parts as follows : 


Carbon 49.2 

Hydrogen 7.8 

Oxygen 24.4 

Nitrogen 17.9 

Sulphur 0 ........... . 0.7 


100.0 

This ossein is insoluble in either hot or cold water. Other 
parts of the animal body, as skin, horn, and connective tissue, 
have the same composition as ossein, and are in all respects 
similar to it. 

Hut when ossein is boiled with water it undergoes a modi- 
fication into the substance called gelatine.^ which has some- 
what difEerent properties, especially in being soluble in warm 
water. 

In the manufacture of gelatine the raw material — usually 
the parings of skins, with hoofs, etc. — is treated somewhat 
differently by different manufacturers. When it is received at 
the factory it is treated with milk of lime and dried in sheds, 
so as to stop the decomposition which would otherwise take 
place. When required for use, the lime washed off, and, 
after exposure to the air for two or three days, the skins, etc., 
are boiled in water until the transformation of ossein into 
gelatine is complete. 

To clarify the hot liquid, either alum or albumen is added, 
which carries the impurities down to the bottom. The insoluble 
parts are then removed from the boilers by a strainer or 
colander, and the liquid gelatine is poured upon tables to 
solidify, the drying being afterwards completed upon nets. 

The gelatine so prepared is a brittle, glassy, transparent 
mass, which swells up in cold, and dissolves in warm water. 
When the solution is cold, if it contains more than one per 
cent, of gelatine, it forms a tremulous jelly. Gelatine is 
insoluble in alcohol or ether ; it is precipitated from its solu- 
tions by the addition of excess of alcohol, or by tannic acid, 


94 


THE CHEMISTRY OF PHOTOGRAPHY. 


corrosive sublimate, or platinic bichloride. Impure gelatine 
may be purified by dissolving it in warm water, allowing it to 
cool, squeezing the jelly so produced through coarse canvas, 
then washing several times in tepid water (which will remove 
the coloring matters), dissolving again in warm water, and 
finally precipitating as a whole clot by the addition of an equal 
quantity of alcohol. 

By long-continued boiling gelatine is changed into a gum- 
like substance called metagelatine^ which is soluble in cold 
water. Boiling with strong alkalies converts gelatine into 
leucine and glycerine, ammonia being given off. 

Chondrin is a very similar substance to gelatine, differing in 
the fact that it is precipitated by alum, acetate of lead, sul- 
phate of iron, and sulphate of copper. 

Gelatine is now of primary importance to the photographer, 
being used as the vehicle which holds the sensitive salts of 
silver on the glass plates. When impregnated with about one- 
sixth its weight of potassium bichromate, the mixture is readily 
affected by light, and is then insoluble in warm water. This 
fact is the foundation of most of the photo-mechanical print- 
ing processes now employed. 

The principal gelatine manufacturers now prepare a special 
article for photographic work, and this is usually of two 
qualities, soft and hard. “ Nelson’s No. 1 Photographic” is a 
good example of the former, and “ Heinrich’s” or ‘‘Coignet’s 
Gold Label ” of the latter. An admixture of the two varieties 
is best for most purposes. 

Isinglass is a superior, and common glue an inferior variety 
of gelatine. In testing gelatine, e^ch sample is made up into 
a ten per cent, solution with water, and allowed to cool in a 
beaker. The beaker is fitted with a lid, through the centre of 
which passes a stout wire having at its lower end (resting on 
the gelatine) a half-inch ball, and at the upper end a small tin 
canister ; shot is poured into each canister until the ball is 
forced into the gelatine, and the weight of shot required in 
each case indicates the comparative strength of the various 
samples. In the dry state, gelatine keeps well, but when 
moist, or in solution, it soon decomposes. 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


95 


Glass. 

Glass is a transparent, hard, brittle, homogeneous solid, 
formed by melting silica (sand or powdered flint) with oxides 
of the alkaline, earthy, or common metals. It is insoluble in 
all acids except hydrofluoric (HF). 

There are four principal varieties of glass : 

(1) Crown Glass, used for glazing purposes; plate glass is 
it variety of this ; chemically, it consists of silicate of soda and 
lime. 

(2) Bohemian Glass — silicate of potash and lime ; this kind 
of glass is hard to melt, and is, therefore, used for tubes which 
have to be strongly heated, as, for instance, those employed in 
the analysis of organic substances. 

(3) Flint Glass or Crystal, containing silicate of potash and 
lead. This is a heavy, lustrous, and easily fusible variety. 
Our common glass tumblers are usually made of lead glass ; 
and it is practically indispensable for the manufacture of 
achromatic lenses. 

(4) Bottle Glass; silicate of soda and lime, colored green by 
the presence of oxide of iron. This is the cheapest and most 
impure variety of glass. 

Ordinary glass is rarely colorless, and its tints are due to the 
presence of small quantities of the oxides of certain metals, 
especially iron, in the sand which is practically an essential 
ingredient in the manufacture of every variety of glass. 

The colors imparted by these oxides are as follows : 

Protoxide of iron (FeO) green. 

Peroxide of iron (FcoOg) brownish-yeliow. 

Protoxide of copper (CugO) ruby. 

Peroxide of copper (CuO) green. 

Sesquioxide of chromium (CugOg) green. 

Oxide of uranium (UO3) greenish-yellow. 

Oxide of cobalt (CoO) blue 

Oxide of silver (AggO). ...... .lemon to orange. 

Oxide of gold (AU3O3). ruby. 

By far the commonest impurity is the protoxide of iron 
(FeO), which stains the glass green. To correct this the 
manufacturer adds a little black oxide of manganese (MnOg), 


96 


THE CHEMISTEY OF PHOTOGRAPHY. 


which, when heated, readily parts with some of its oxygen. 
This released oxygen unites with the protoxide of iron, raising 
it to the state of peroxide (FegOg), which imparts only a light 
lemon tint to the glass — a tint which is practically invisible. 
But, unfortunately, an excess of oxide of manganese is almost 
always added, and under the influence of light this colors the 
glass a pink or puce color. This is a frequent cause of studios 
becoming ‘^slower” — exposures lengthened — after they have 
been erected for some years. On taking out an old pane of 
glass the difference in color between “ that which has been 
exposed to the light and that which has been protected by the 
rabbet will often be very noticeable.” Fortunately it has been 
discovered that arsenic trioxide (AsgOg) will oxidize the iron 
as effectually as manganese ; and as all the arsenic dissipated 
by heat passes up the chimney of the glass furnace, it leaves 
no injurious residue. 

Lead glass may be known by its blackening all through 
when heated strongly in a gas flame. 

Plate glass is made by pouring melted glass upon a level 
iron table, and rolling it out to the required thickness with 
iron rollers ; it is then ground and polished. 

Sheet glass is made by ‘^blowing” the glass into large 
cylinders, which are then cut with a diamond and again heated 
till they open out into flat sheets. “ Patent plate ” is only 
sheet glass which has been ground and polished. For large 
negatives (say sizes above whole-plate) patent plate is to be rec- 
ommended, as it is, or should be, perfectly flat ; and there is 
thus little danger of breaking the negatives in the printing 
frame. 

The ^‘ruby” glass so largely used by photographers is 
made by “ flashing ” {i.e.^ coating) wdiite glass with a thin layer 
of glass containing protoxide of copper. Glass coloied all 
through is called “ pot metal,” and pot metal colored ruby by 
protoxide of copper is better than the ‘‘flashed” glass. But 
the safety (for photographic purposes) of any sample of red or 
yellow glass can only be properly tested by the spectroscope, 
as some varieties of red glass allow many blue rays also to 
pass. 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


97 


Glycerine. 

Formula, CgligOg: Combining weight, 92. 

Glycerine is largely obtained, as a bye-produet, in the manu- 
facture of soap. For when a fatty body is boiled with a 
caustic alkali w^e get soap and glycerine. It is also produced 
in W ilson’s patent process for candle-making, by wdiich fat is 
decomposed by superheated steam. Glycerine is a viscous, 
colorless liquid, with a very sweet taste, but no smell. It 
mixes readily with water, and is neutral to litmus paper. 
Glycerine is sometimes added to the pyro developer, wdiich it 
assists in preserving ; it also acts as a mechanical resti ainer, 
preventing tlie too rapid decomposition of the silver sub- 
bromide or bromide. 

Gold Cyanides. 

When a solution of potassium cyanide is added to a dilute 
solution of gold trichloride, a yellow precipitate of gold 
cyanide Au(Cds) is produced. The principal solvent of this 
substance is potassium cyanide in excess, wdiich combines with 
it to form a double salt — potassium-gold cyanide — which is 
largely used for gilding by means of the galvanic battery. 
Cupper and silver articles may be gilt by simply making them 
perfectly clean and then dipping them into the liquid. 

Gold and Sodium Chloride. 

Formula, NaAuCl 4 + 2 H 3 O : Combining w'eight, 

361 + 36 = 397 . 

Prepared by dissolving common salt in a solution of gold 
trichloride, and evaporating the solution. Yellowdsh-red 
crystals of the double salt then appear. When exposed to the 
air, these crystals effloresce and become yellowL When 
anhydrous they are red. This salt is also known as sodium 
chloro-aurate. Prepared in this way, the gold salt keeps 
better (7.^., is less deliquescent) than if in the form of the 
pure chloride. When used for toning purposes, a rather 
larger quantity, by weight, than of the pure gold chloride 
will, of course, be required. 


98 


THE CHEMISTRY OF PHOTOGRAPHY. 


Gold. 

Symbol, Au : Combining weight, 192. 

Gold is found either in detached grains or nuggets scattered 
through sandy or alluvial deposits, or disseminated in veins or 
reefs of quartz. Native gold usually contains a little silver. 
California and Australia yield nine-tenths of the gold now 
raised annually. 

Gold is yellow, lustrous, soft, very malleable and ductile. 

It reflects yellow light, but very thin gold-leaf transmits green 
light. Neither oxygen, air, nor steam have any effect upon 
gold, and it is unaffected by acids, except the mixture of nitric 
and hydrochloric acids known as aqua regia, in which it readily 
dissolves to form trichloride of gold, AuClg. 

Gold is too soft for use alone, so that for coins, jewelry, 
etc., it is alloyed with either copper or silver, or both Pure 
gold is 24 carats fine, standard gold (employed for coinage) 
22 carats, and 18, 15, 12 and 9 carat gold are also recognized. 
These expressions mean that 24 parts by weight of the alloy 
contain 22, 18, 15, 12 and 9 parts by weight of pure gold • 
respectively. In the German, American and Italian coinage 
the standard is 21.6 carats only. English gold coins consist of 
11 parts of pure gold alloyed with 1 part of copper. 

Gold is precipitated from its solutions by the addition of 
ferrous sulphate. It then appears as a brown powder, fusible 
under the blow-pipe, Oxalic acid also slowly reduces gold. 

Gold Trichloride. 

Formula, AuClg: Combining weight, 302^, 

There are two chlorides of gold, the mono-chloride, AuCl, 
and the terchloride, or trichloride, AuClg; it is the latter 
which is exclusively employed in photographic operations. 
With pure gold there is no difficulty in the preparation of this 
salt. It is simply necessary to dissolve the gold in about eight 
times its weight of aqua-regia, an operation which is facilitated 
l)y gentle warmth, as by placing the glass vessel in hot water. 
AVhen the gold is entirely dissolved, the solution must be 
poured into a small porcelain crucible, and evaporated until 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


99 


the free acid is all, or nearly all, driven off ; a few drachms of 
distilled water may then be added, and the evaporation con- 
tinued a little longer Lastly, enough distilled water must be 
added to bring the solution to a standard strength, say one 
grain of gold to three drachms of water. 

Thus, if 20 grains of pure gold were used, enough water 
should be added to make the whole up to 7|- ounces. The 
solution of gold trichloride so prepared has a yellow color, and 
is slightly acid. It should be kept in an opaque bottle, or in 
a dark place, as light causes it to be decomposed, the gold 
separating as a brown or black powder. 

AuClg combines with alkaline chlorides to form double salts 
called chloro-aurates ; thus we have sodium chloro-aurate JN’aCl, 
AUCI3+2H2O, and potassium chloro-aurate KCl,AuCl3 + 
2H2O. These are yellow crystalline substances, and it is in 
this state that chloride of gold is usually sold to photograph- 
ers, the salts being sealed up in small glass tubes. 

It is not at all difficult to make gold trichloride, and a con- 
siderable saving may be effected by those who can prepare it 
for themselves. Pure gold can be obtained by dissolving the 
substance containing the precious metal in aqua-regia, diluting 
with water, and then sodding ferrous sulphate, wdiich will pre- 
cipitate the gold as a brown powder, while the other metals 
will remain in solution. When the brown powder has all 
settled to the bottom (which takes a long time) the liquid 
must be poured off, and the gold first w^ashed with distilled 
water, and then re-dissolved in fresh aqua-regia. The opera- 
Lon can then be continued as described above. But for the 
purpose of toning prints — the only process in which gold 
trichloride is required by the ordinary photographer — pure 
gold is not absolutely necessary, and gold coins may be used 
without injury to the result. Australian coins are the best, 
because they contain less copper. Or almost any scraps of 
broken gold ornaments may be used, and the copjier with 
which the gold is alloyed may either be removed or allowed 
to remain, as its j^resence seems to make no difference wdiat- 
ever to the action of the toning solution. A sovereign weighs 
— or should weigh — 113 grains, and will readily dissolve in 


100 


THE CHEMISTRY OF PHOTOGRAPHY. 


about ten drachms of hot aqua-regia. IN^ow evaporate the solu- 
tion down to about four or five drachms (this should be done 
where there is a good draught, as the acid fumes are injuri 
oils), add a little chalk or whiting to neutralize the remaining 
acid, and filter off the sediment produced ; lastly add distilled 
water to make the whole up to 7|- ounces. The result will be 
a solution containing 174 grains of terchloride of gold, or two 
grains of metallic gold to each drachm. As a rule, one grain of 
gold is sufficient to tone a sheet of sensitized paper. The same 
quantity of gold purchased in the usual small tubes would 
have cost 23 shillings (English), besides which there would 
have been some uncertainty as to getting the true weight and 
the pure article. 

Gold and Sodium Hyposulphite. 

Formula, AuHa 3 S 40 g + 2 II 2 O: Combining weight, 

489 + 36 = 525. 

Prepared by gradually mixing concentrated solutions of 
gold trichloride and sodium hyposulphite, in the proportion 
of three parts of the former to one part of the latter salt, and 
then adding alcohol, which precipitates the double hyposul- 
phite of gold and sodium in the form of delicate, colorless 
needle-like crystals. It has a sweetish taste, and is soluble in 
water. This substance was formerly known as sel d’or,” and 
wvas used to tone the daguerreotype plates employed in the 
early days of photography. 

The silver plate bearing the picture to b toned was covered 
with a solution of sel d’or.” and was then heated. The 
donide salt w^as decomposed by the heat, the gold being 
deposited upon the picture, to which it gave a pleasing color 
and enhanced durability. Afterwards sel d’or ” was much 
used for toning silver prints on paper, and we have seen some 
of these more than twenty years old which still retained their 
pristine colors. 

Gums. 

Gums are vegetable exudations wliich differ from resins in 
being soluble in water. Gum arabic may be taken as the type 
of “ gums ” generally, its formula being CigllggOig. Gum 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


101 


tragacantli is not a true gum, but a mucilage which differs 
from gum in refusing to dissolve in water, merely swelling up 
and gelatinizing. 

Gums are sometimes used to mount photographs with, but 
for this purpose they are inferior to either starch-paste or the 
alcoholic solution of gelatine. In the dry collodion process a 
weak solution of gum was frequently flowed over the fllm to 
act as a preservative ” or organifler.” It was liable, how- 
ever, to produce a blistering during or after development. 

Gum Dammar. 

. A resin obtained from India and the East Indies. Soluble 
in alcohol, turpentine, benzole, etc. Used in several varnishes, 
and as a retouching medium. 

Hydriodic Acid. 

Formula, HI : Combining weight, 128. 

This is a colorless gas, having a pungent smell, and fuming 
when in contact with air. It is very soluble in water, and is 
readily decomposed by heat. The gaseous acid is prepared by 
jilacing water, potassium iodide, and iodine in a flask, then 
dropping in small fragments of phosphorus, and heating 
gently. If an aqueous solution of the acid only be required, 
the readiest mode is to pass sulphuretted hydrogen through 
water in which iodine is suspended, and Alter off the sul| 3 hur 
which is liberated. 

Hydrochloric Acid. 

Formula, HCl : Combining weight, 36^. 

Pure HCl is a gas, usually prepared by acting on common 
salt with slightly diluted sulphuric acid. It is very soluble in 
water. Commercial hydr.ichloric acid has a yellow tint, owing 
to the presence of a little iron ; the pure aqueous solution is 
colorless. It fumes when in contact with moist air ; and if a 
glass rod be dipped in liquid ammonia, dense white fumes are 
seen when it is brought near HCl. But the best test for HCl, 
or any soluble chloride, is the white precipitate of silver 


102 


THE CHEMISTRY OF PHOTOGRAPHY. 


chloride, which is produced bj the addition of a drop of a 
solution of silver nitrate. This precipitate is insoluble in 
pitric acid, but soluble in ammonia. 

Hydrochloric acid was formerly known as muriatic acid. 

A weak solution of HCl (or, better, of HCl and alum) is 
very useful as a clearer,” removing the brown stain produced 
by the action of the pyro developer. 

Hydrocyanic Acid (Prussic Acid). 

Formula, HCA or HCy : Combining weight, 27. 

Hydrocyanic acid is contained in bitter almonds, laurel 
leaves, etc., and it can be extracted from them by distillation. 
It is usually prepared by heating dilute sulphuric acid with 
potassium cyanide in a retort. It is intensely poisonous, so 
that it is dangerous even to inhale its vapor, and the greatest 
care should be used in experimenting with it. Hydrocyanic 
acid was discovered by Scheele, in 1782, and was long known 
as prussic acid. It cannot be kept for any length of time, but 
turns brown and decomposes; its odor is very characteristic, 
resembling that of peach blossoms, or oil of bitter almonds. 

Hydrobromic Acid. 

Formula, HBr: Combining weight, 81. 

For laboratory purposes HBr is prepared by dropping 
bromine into water containing fragments of phosphorus. It 
is a colorless gas, very soluble in water. 

Hydrofluoric Acid. 

Formula, HF : Combining weight, 20. 

Prepared by decomposing fluor-spar with sulphuric acid, in 
platinum or lead vessels. HF is a colorless liquid whose most 
remarkable property is its power of corroding or etching glass. 
It must be kept in gutta-percha bottles, or the dilute acid may 
be preserved in glass bottles coated inside with paraffin. The 
divisions on thermometers, glass measures, etc., are usually 
produced by coating the surface with paraffin, scratching off 
the parts required with a steel point, and then submitting the 


CHEMICALS EMPLOYED IN PHOTOGEAPHY. 


103 


glass to the action of vapor from hydrofluoric acid placed in a 
leaden trough. 

A dilute solution of HF — about 1 to 20 of water — cleanses 
glass bottles and plates very effectively. The strong acid is so 
corrosive that it burns the skin dangerously should it come 
in contact with it; death has been caused by inhaling the 
fumes. 

A ready method of using this acid to mark glass is to rub up 
equal parts of barium sulphate and ammonium fluoride in a 
mortar, adding enough HF to make a paste. Place the whole 
in a leaden or gutta-percha cup (an egg-cup paraflined over will 
answer) and add more acid till it is of the consistency of 
cream. The mixture may now be used with a quill-pen just 
like ink, leaving it a few minutes on the glass before washing 
off. 

Marks upon glass produced by the action of HF have the 
great advantage over labels of being indelible. Another use 
of this substance is that a very weak solution enables gelatine 
films to be readily stripped from glass. 

Hydrochinone. 

Formula, CgHgOgi Combining weight, 110. 

Hydrochinone — whose name has been spelt in many 
different ways, as hydroquinone, hydrokinone, etc. —is also 
known as quinol. Like pyrogallol, it is a benzine derivative, 
and, indeed, it only differs in chemical composition from that 
M^ell-known substance in containing one atom less of oxygon 
(Pyro = 051-1^03). II vd rocliinone occurs naturally in the 
leaves of the arbutus and certain allied plants. Formerly it 
war prepared from quinic acid (C^H^g^e) converting 

the latter into kinone (Cgll^Og), and then treating the kinone 
with a reducing agent, such as sulphurous acid. It is now 
obtained far more cheaply by preparing the kinone from 
aniline by the action of sulphuric acid and potassium bichro- 
mate. Hydrochinone forms hexagonal, colorless or slightly 
yellowish crystals, which are soluble in water, alcohol or ether. 
It is inodorous, has a sweetish taste, and readily fuses. 


104 


THE CHEMISTRY OF PHOTOGRAPHY. 


Ill photography hydrochirione was introduced by Abney as 
a developer in 1880. Its jirincipal advantage over pyrogallic 
acid is in the fact that it discolors the gelatine plates very 
little, not absorbing oxygen from the atmosphere so 
readily. It is suitable for developing either silver bromide 
or silver chloride tilms, and for instantaneous work it 
is especially useful. It does not require the presence of any 
restrainer, such as ammonium or potassium bromide, and 
potassium carbonate accompanies it as an accelerator better 
than ammonia. It should be kept dry, and mixed as required 
to a strength of from two to four grains per fluid ounce of 
developer. 

Ilydrochinone was first prepared by Caventon and Pelletier^ 
in 1820. It melts at 336 deg. F. The substance known as 
“Permanent Hydroquinone ” is sold in lemon-yellow crystals, 
and contains from ^ to ^ per cent, of sulphurous acid. 

Hydro-Sulphuric Acid — Sulphuretted Hydrogen. 

Formula, HgS : Combining weight, 34. 

Hydro-sulphuric acid is certainly better known under its 
familiar name of sulphuretted hydrogen — or “ rotten-egg gas.” 
It is almost always prepared by acting on iron sulphide with 
dilute sulphuric acid, but the operation should never be con- 
ducted in the dark-room, as the gas attacks the silver salts used 
by ])hotographers. 

The most valuable property of sulphuretted hydrogen is its 
power of combining with, and precipitating as insoluble 
sulphides, certain of the metals, among which is silver. For 
this reason photograjDliers use it to recover silver from their 
residues. The H 2 S gas is allowed to bubble through the 
vessel containing the waste liquids, when any silver which may 
be present falls to the bottom as a black powder — sulphide of 
silver. This is removed, dried and fused, when metallic silver 
is obtained. 

Water absorbs about three times its volume of sulphuretted 
hydrogen, and the solution may be used instead of the gas. It 
has a poisonous effect when breathed. 


CHEMICALS P:MPL0YED IN PHOTOGKAPHY. 


105 


Hydroxyl — Peroxide of Hydrogen. 

Formula, II Combining weight, 34. 

Hydroxjl is now prepared by dissolving moist hydrated 
barium peroxide in dilute sulphuric acid, filtering and evapo- 
rating in vacuo with sulphuric acid. As so obtained it is a 
colorless, syrupy liquid. It is remarkable in that it is both an 
oxidizing and a reducing agent. In the former capacity it 
converts black plumbic sulpliide (PbS) into white plumbic 
sulphate (PbS 04 ), and is, therefore, useful for cleaning oil 
])aintings in which the white lead has become discolored by 
the sulphurous fumes from gas, etc. It also bleaches organic 
matters, changing the color of dark hair to yellow, so that, 
under the name ot “ auricomas^^ etc., it is used as a hair-dye. 

But hydroxyl is also a reducing agent, depriving certain 
compounds of the whole or part of their oxygen, when brought 
into contact with them. In this way it decomposes silver 
oxide forming silver, water, and oxygen. 

In photograph}", hydroxyl is used for removing the last 
traces of “hypo’' from negatives and prints, which it does by 
oxidizing the hurtful hyposulphite into the harmless 
sulphate. Care must be taken, however, not to use too strong 
a solution, or to leave the objects in too long, or reduction will 
take place. 

IIVDROXYLAMINE. 

Formula, Is II 3 O; Combining weight, 33. 

This compound, which may be considered as ammonia in 
which one atom of hydrogen is displaced by a compound 
atom of hydroxyl, has been prepared from nitric acid by the 
action of tin and hydrochloric acid. It is a powerful base, 
and one of its compounds — the hydrochloride of hydroxy- 
lamine* — has been proposed by Messrs. Spiller and Egli as a 
developing agent. Its cost is at present much greater than 
])yrogallic acid, but it has a great advantage in that it does not 
slain the gelatine plates. Dr. Divers has prepared hydro- 
chloride of hydroxylamine by the direct action of hydro- 


* Also known as hydroxylamine hydrochlorate ; its formula is NH^OHCl. 


106 


THE CHEMISTRY OF PHOTOGRAPHY. 


chloric acid on fulminating mercury, but the process requires 
to be conducted with great care, and is not one to be practised 
by the ordinary worker in photography. Still this is the less 
necessary, as the substance may now be obtained commercially. 
It seems speciall}^ suited for the development of gelatino- 
chloride films. Its chief drawback is a great tendency to 
cause “ frilling.” 

IIypochlorous Acid. 

Formula, HCIO: Combining weight, 52|-. 

Only the aqueous solution can be obtained, which is com- 
monly effected by distilling a mixture of one part of nitric 
acid with two parts of bleaching powder, or by shaking chlo- 
rine water up wfith precipitated mercuric oxide. The solution 
so obtained is a yellow liquid which possesses powerful oxidiz- 
ing properties. It also converts silver oxide into silver chlo- 
ride, oxygen being evolved. 



CHAPTER XIII. 


CHEMICALS EMPLOYED IN PHOTOGRAPHY 
(CONTINUED). 

Iodine. 

Symbol, I : Combining weight, 127. 

The elementary body iodine was discovered by Cunrtois, at 
Paris, in 1812. He obtained it from kelp — the ashes of cer- 
tain seaweeds which contain the iodides of sodium and 
magnesium. Iodine is prepared in a precisely similar way to 
bromine and chlorine, its fellow-halogens, by heating an 
iodide — usually potassium iodide — with sulphuric acid and 
black oxide of manganese. Iodine is slightly soluble in 
water, more so in alcohol. A few drops of tincture of iodine 
added to the hydroquinone developer have a powerful accel- 
erating effect, and reduce contrasts. 

SKI + MnOg + 2113804= 13^ K3S04+MnS04 + 2H30. 

Iodine is usually seen as bluish-black scales, having a some- 
what metallic lustre. It melts at 239 deg. Fahr., and at 4u0 
deg. Fahr. is converted into a beautiful violet-colored vapor. 
Iodine is but very slightly soluble in w^ater, but readily dis- 
solves in alcohol, in carbon-bisulphide, or in chloroform. 
Free iodine forms a blue compound with starch, and 
this furnishes a well-known test. To make this test, a drop 
of potassium iodide solution is added to some very 
dilute starch-paste. If a drop or two of chlorine water 
is then added to the mixture, some iodine will be liberated, 
and will unite with the starch to form a blue compound. 
This blue compound, once formed, is itself a delicate test for 
hypo,” the latter substance discharging the blue color. A 
solution of iodine in water can be obtained, if to the water is 
hrst added some iodide of potassium. This solution has been 


108 


THE CHEMISTRY OF PHOTOGRAPHY. 


recommended by Yogel and by Chapman Jones as a hypo-elimi- 
nator. After fixing as usual, wash the prints in three or 
four changes of water, and then ])lace them in water colored by 
the iodine solution to about the tint of pale sherry, and replace 
this iodine water with fresh as may be necessary, until the 
prints show a slight but persistent blue color. This blue is 
especially visible on the back, and shows excess of iodine, and 
therefore absence of hypo. To got rid of the blue color, rinse 
the prints in a solution of sulphite of soda and carbonate of 
soda, made very weak indeed (a few drops of a strong solution 
to a pint or a quart of water), and then w^ash them in two or 
three changes of clean watei*, and dry. The iodine solution 
must not be used in a metallic vessel.” 

Iridium Tetra-chloride. 

Formula, IrCl4: Combining weight, 335 . 

Iridium is an intensely hard metal which is found in small 
quantities mixed with platinum. It is now used to make the 
indestructible points of pens. There are three chlorides of 
iridium, but the most interesting is the tetra-chloride, 
which is produced by dissolving finely divided iridium in 
aqua regia, heating, and evaporating to dryness. The IrCl4 so 
obtained is a black, deliquescent, amorphous substance which 
dissolves in water, forming a reddish-yellow solution. It com- 
bines with ammonium to form ammonium chlor-iridiate. An 
aqueous solution of the latter salt is naturally of a pale-yellow 
C(dor, but when exposed to light it becomes white, and then 
changes to purple, violet, and lastly assumes a beautiful blue 
tint. 

Iron Acetate. 

Formula, Fe(C3ll3 02)2 4-4II2C : Combining weight, 246 . 

There are two acetates of iron, ferrous acetate, which has 
the formula given above, and ferric acetate, Fe2(C2lT302)6. 
It is the ferrous salt — the protacetate of iron — which is used 
in photography. 

When iron is dissolved in acetic acid and the solution evapo- 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


109 


rated in vacuo ^ greenish-white crystals of ferrous acetate, very 
soluble in water, are produced. It can also be obtained by 
acting on sugar of lead (lead acetate) with carbonate of iron ; 
or by the combination of ferrous sulj)hate (green vitriol) and 
calcium acetate. Under the name of black liquor,” or “iron 
liquor,” ferrous acetate is largely used as a mordant in calico- 
printing. Dr. Just (in his book on “Gelatine Emulsion 
Papers,” 1890) recommends ferrous acetate as a good devel- 
oper for gelatirio-chloride papers. 

Iron Ammonium Citrate. 

Prepared by dissolving two parts of freshly precipitated 
ferric hydrate in three parts of citric acid, and passing 
ammonia through the mixture until it is saturated. On 
evaporating, a yellowish mass of ammonia-citrate of iron will 
be obtained, which is insoluble in strong, but soluble in weak 
alcohol of 40 per cent. 

Iron Ammonium Sulphate. 

Formula, FeSO^, (1^114)2 SO4 +6II3O : Combining weight, 

284 + 108 = 392. 

Prepared by dissolving 38 parts of ferrous sulphate with 33 
parts of ammonium sulphate in the minimum quantity of hot 
water. When the solution is filtered and allowed to crystal- 
lize it forms transparent bluish-green crystals, which are 
soluble in five parts of cold or two of hot water. It is a very 
stable substance, and hence is frequently used instead of 
ferrous sulphate for analytical purposes. 

(Ikon): Ferric Nitrate. 

Formula, Fe3(N03)g: Combining weight, 484. 

Ferric nitrate is formed by dissolving iron in nitric acid. On 
evaporating the solution it deposits colorless crystals which 
contain a large amount (12 or 18 molecules) of water of 
crystallization. These crystals deliquesce rapidly in air, and 
dissolve in water to form a brown liquid, which is decomposed 
by boiling. 


no 


THE CHEMISTRY OF PHOTOGRAPHY. 


(Iron): Ferric Oxalate. 

Formula, Fe 2 (C 204 ) 3 : Combining weight, 376. 

Prepared by dissolving ferric hydrate (Fe 2 (HO)g) in a 
solution of oxalic acid. It is very soluble in water. Its use in 
photography depends mainly on the fact that by exposure to 
light it is reduced to ferrous oxalate. The paper employed 
in the platinotype printing process is prepared with ferric 
oxalate. 

(Iron): Ferric Sulphate. 

Formula, F 2 (S 04 )g + 9 H 2 O: Combining weight, 

401 + 162 — 566. 

Ferric sulphate can be prepared by oxidizing ferrous sul- 
phate with nitric acid. Ten parts of the ferrous salt are 
dissolved in water with four parts of sulphuric acid, and nitric 
acid is then added to the hot solution. On evaporation, the 
anhydrous ferric salt is obtained as a white powder. 

(Iron) : F errous Bromide. 

Formula, FeBr 2 -+ 6 H 2 O: Combining weight, 

216 + 108 = 324. 

A solution of ferrous bromide can be made by dissolving 
iron in hydrobromic acid. By evaporating this solution, green 
crystals of the salt can be obtained. 

(Iron) : Ferrous Chloride. 

Formula, FeCl 2 . Combining weight, 127. 

Prepared by dissolving iron in hydrochloric acid and 
evaporating in vacuo ^ when bluish-green crystals having the 
composition FeCl 2 + 4 Il 20 are obtained. These deliquesce 
and decompose in air. They are very soluble in water and in 
alcohol. 

(Iron) : Ferrous Iodide. 

Formula, Fel 2 + 4 Fl 20 : Combining weight, 310 + 72 = 382. 

Prepared by digesting iron tilings and iodine in water. 
When the colorless aqueous solution so obtained is exposed to 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


Ill 


air it decomposes ; but this may be prevented by the addition 
of a little sugar. On evaporation, green crystals of ferrous 
iodide containing four equivalents of water are obtained. The 
anhydrous salt can be obtained by heating iron filings with 
iodine in a closed porcelain crucible. 

(Iron): Ferrous Titrate. 

Formula, re(H 03 ) 2 +G rigO: Combining weight, 

180 + 108 = 288. 

Prepared by adding barium nitrate to ferrous sulphate and 
evaporating in vacuo. The crystals so obtained are very 
soluble in water. This salt is very unstable, quickly absorb- 
ing oxygen and passing into ferric nitrate. 

(Iron Perchloride) : Ferric Chloride. 

Formula, FegCl^: Combining weight, 325. 

Perchloride of iron, or ferric chloride, is obtained by dis- 
solving peroxide of iron (FogOg) in hydrochloric acid. The 
solution is yellow when dilute, reddish-brown when concen- 
trated. By passing chlorine over red-hot iron wire, brilliant 
red crystals of anhydrous perchloride of iron are produced. 
These are very soluble in water, alcohol, or ether. 

(Iron Protosulphate) : Ferrous Sulphate. 

Formula, FeS 04 + 7H2 0: Combining weight, 

152 + 126 = 278. 

Protosulphate of iron, or ferrous sulphate, is commercially 
known as copperas, or green vitriol. The pure salt can be 
obtained by dissolving pure iron wire in sulphuric acid, but it 
is made on a large scale by exposing heaps of moistened iron 
pyrites (FeSg) to the action of the air. As usually seen it 
consists of green crystals, readily soluble in water, almost 
insoluble in alcohol. All the ferrous compounds combine 
readily with oxygen, and when ferrous sulphate is left in con- 
tact with air, as in a partly filled bottle of the aqueous solu- 
tion, the salt absorbs oxygen and is converted into ferric 
sulphate. The change is indicated by the alteration of color 


112 


THE CHEMISTEY OF PHOTOGRAPHY. 


from green to yellovvisli-brown. It may be retarded or pre- 
vented by adding two or three drops of sulphuric acid, or by 
keeping a little clean iron wire in the bottle. The bottle 
should also be kept quite full and well corked. 

Ferrous sulphate was introduced as a developing agent by 
Eobert Hunt, in 1844. It was commonly employed both in 
the calotype and in the collodion process. As an ingrediei^t 
for making the ferrous oxalate developer, ferrous sulphate is 
still largely used. 

Kaolin (China Clay). 

Formula, AlgSioO^ + 2 H 2 O: Combining weight, 258. 

Kaolin, or China clay, is a silicate of alumina, produced by 
the disintegration of the feldspar which is an essential ingre- 
dient of all granites. It is an extremely fine white powder, 
which is frequently used to clear, or decolorize, solutions of 
nitrate of silver, such as the negative bath in the wet collodion 
process, which have become brown, owing to the presence of 
albumen or other organic matter. It is a natural product, 
known also as China clay or porcelain clay, occurring plenti- 
fully in regions where granitic rocks abound. 

Lead Acetate. 

Formula, Pb(C2ll303)2 + 3 II 2 O: Combining weight, 878. 

Prepared by dissolving litharge (lead monoxide) in acetic 
acid. From its appearance and its sweet (though also 
astringent) taste, lead acetate is commonly known as sugar of 
lead. The crystals are soluble in a little more than their own 
weight of water ; soluble also in alcohol. The aqueous solu- 
tion is frequently milky, from the presence, or formation, of 
lead carbonate. Lead acetate was found a useful addition, by 
the early experimenters, to the gallic acid they used for 
developing pictures on paper. By its use the details of the 
picture were brought out more rapidly and more clearly. 
Lead acetate has also been recommended as a hypo eliminator. 
Added to the ordinary fixing bath it gives prints a blackish 
tone. 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


113 


Lead Chloride. 

Formula, PbClgi Combining weight, 2-12. 

Prepared by dissolving lead oxide or carbonate in hydro- 
chloric acid. It is but slightly soluble in cold, though more 
so in hot, water, from which (on cooling) it is deposited in 
white silky needles. 

Lead Ferrocyanide. 

Formula, Pl) 2 Fe(CN)g +31120 : Combining weight, 

624 + 54 = 678. 

A white precipitate of lead ferrocyanide is formed when a 
solution of potassium ferrocyanide is mixed with one of lead 
nitrate. The salt parts with its water of crystallization when 
heated. It is insoluble in water ; partly soluble in hot ammo- 
nia ; very soluble in a hot solution of ammonium chloride. 

Lead Nitrate. 

Formula, Pb(N 03)2 : ' Combining weight, 3304. 

Prepared by dissolving litharge (PbO) in hot dilute nitric 
acid. On cooling and evaporating, milk-white octahedral 
crystals of lead nitrate are obtained. It is soluble in water ; 
but very slightly soluble in alcohol. 

Lithium Bromide. 

Formula, LiBr : Combining weight, 87. 

Prepared by dissolving lithium carbonate, or lithia (LigO) 
in hydrobromic acid. It is very soluble in water or alcohol. 

Lithium Carbonate. 

Formula, Li g CO 3 : Combining weight, 74. 

Metallic lithium was discovered by Arfvedson in 1817. The 
carbonate is made by adding ammonium carbonate to lithium 
chloride. It is only slightly soluble in water ; but the 
solution acts as a powerful accelerator to pyrogallol as a 
developer. 


114 


THE CHEMISTRY OF PHOTOGRAPHY. 


Lithium Iodide. 

Formula, Lil + SHgO: Combining weight, 134 + 54 = 188. 

The elementary body, lithium, was discovered in 1817, 
though the pure metal was not isolated till 1855, by Bunsen. 
It is the lightest known solid. Although very rare in any 
quantity, yet minute traces of the salts of lithium occur almost 
everywhere in water, soil, animals and plants. The principal 
lithium compound which has been used in photography is 
lithium iodide, which may be obtained by dissolving lithium 
hydrate or carbonate in hydriodic acid. Another method is 
to mix strong solutions of calcium iodide and lithium sulphate, 
evaporate to dryness, and treat the residue with alcohol, which 
will dissolve out the lithium iodide. The long, slender 
crystals of lithium iodide are so very deliquescent, and the 
pure salt is so expensive, that it has not come into use for 
iodizing collodion, for which it is otherwise well suited, being 
more readily soluble in alcohol than the iodide of potassium 
generally em^iloyed. 

Magnesium. 

Symbol, Mg : Atomic weight, 24. 

Metallic magnesium is a silverjq lustrous metal, which soon 
tarnishes in moist air. It is manufactured in large quantities 
from the chloride, and is chiefly sold as ribbon,” ‘‘ wire,” or 
in the powdered state. It has a great affinity for oxygen, and 
when it is ignited (which may be effected by simply holding 
it in the flame of a candle) it produces a bluish-white light of 
dazzling brilliancy. This magnesium light is very rich in 
actinic rays, and hence it has been largely used for photo- 
graphing dark interiors, as caves, etc., and for photography at 
niglit. Bunsen and Boscoe found that while the light-giving 
value of direct sunlight is 524 times greater than that of burn- 
ing magnesium, its chemical value is only 36 times as great. 
A burning magnesium wire about one-twenty-sixth of an inch 
in thickness gives as much light as 74 stearin candles weighing 
live to the pound. 

A convenient lamp is sold which pushes out the metallic 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


115 


ribbon as fast as it is consumed, and so maintains a fairly 
constant light. A substitute for this is to use a few inches of 
a narrow tin tube — a pea-sliooter, for instance — and pass the 
ribbon through it. The tube can be held in the haiid, and the 
ribbon pushed through steadily while it burns at the far end. 
The white smoke produced is wMgnesia^ i.e.^ magnesium oxide. 

The latest development of the magnesium light in photog- 
raphy is its use in the form of powder, either alone or spread 
upon gun-cotton, or mixed with oxidizing substances, such as 
chlorate of potash. Fifteen grains of the powdered metal 
intimately mixed with half its weight of gun-cotton, and burnt 
at a distance of six or eight feet from the sitter, will give a 
flash of such brightness that an instantaneous portrait can be 
readily secured. 

Magnesium Bromide. 

Formula, MgBrg: Combining weight, 184. 

This substance is found in sea water and in saline springs. 
It is deposited as needle-shaped crystals, having the composi- 
tion MgBi-g+dHgO, when magnesia is heated in hydrobromic 
acid. By heat, these crystals are decomposed into the 
substances from which they were produced. 

Magnesium Carbonate. 

Formula, MgCOgi Combining weight, 84. 

Magnesium carbonate occurs in nature as the mineral called 
magnesite. It is soluble in water saturated with carbonic acid 
gas, the solubility increasing rapidly with the pressure. 

The magnesia alba of druggists is a mixture of several 
complex carbonates of magnesium. It is a bulky white 
powder soluble in ammoniacal solutions. 

Magnesium Chloride. 

Formula, MgCla : Combining weight, 95. 

Prepared by evaporating magnesia dissolved in hydro- 
chloric acid to which an equal weight of sal-ammoniac 
has been added, and then fusing the mixture. It is a 


116 


THE CHEMISTEY OF PHOTOGRAPHY. 


white deliquescent substance, very soluble in water; all but 
insoluble in alcohol. Magnesium chloride is used in preparing 
chloride of silver emulsions; it has also been tried as a fixing 
agent for silver prints, in the place of hypo. 

Magnesium Iodide. 

Formula, Mglgi Combining weight, 278.. 

This substance occurs in sea water, and in brine springs. 
It can be prepared by dissolving magnesia in hydriodic acid. 
It forms crystals whicli deliquesce in air, and decompose when 
heated, iodine being liberated. 

Magnesium Nitrate. 

Formula, Mg(N03)2 + 6II2O : Combining weight, 

148 + 108 = 256. 

This salt occurs in the mother-liquor from the saltpetre 
manufacture. It can be prepared by dissolving magnesia alba 
in nitric acid. Its prismatic crystals deliquesce in air. They 
are soluble in half their weight in water; soluble also in 
alcohol. 

Magnesium Sulphate. 

Formula, MgS04 + 7H2O : Combining weight, 

120 + 126 = 246. 

Magnesium sulphate is familiarly known as ‘‘ Epsom salts, 
from its occurrence in the water of a mineral spring at Epsom. 
It is now chiefly obtained from a mineral called Jceiserite, 
which occurs in layers in the salt-beds at Stassfurt. It i& 
usually sold as a white crystalline powder, which is very 
soluble in water, but insoluble in alcohol. 

The addition of Epsom salts to the fixing-bath is found to 
check, or prevent, the ‘‘frilling” to which certain dry-plates 
are more or less subject. 

Mercury (Quicksilver). 

Symbol, IJg : Combining weight, -200,, 

Mercury, whose symbol, Ilg, is derived from the name 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


117 


hydrargijrum applied to it by Pliny, is commonly called quick- 
silver. It is found, combined with sulphur, as cinnabar 
(mercuric sulphide), in the famous mines of Almaden in 
Spain, Idria in Carniola, and in California. The mercury of 
commerce usually contains small quantities of other metals, 
as iron, lead and zinc. It may be freed from these by distil- 
lation, or by treatment with dilute nitric acid. 

Pure mercury is a silvery-white metal, freezing at — 103 
deg. Fahr., and boiling at 575 deg. Fahr. It combines with 
many of the other metals to form alloys which are called 
amalgams. 

Mercury volatilizes at all temperatures, but of course the 
rapidity of volatilization increases as the temperature 
increases. It is on this fact that its use as a developer in the 
daguerreotype process depends. The exposed silver plate is 
placed over a dish of warm mercury, and the latter metal 
combines with those parts of the plate which have been 
affected by light. 

[The volatilization of mercury at ordinary temperatures 
may be well shown by putting a little mercury in a test-tube, 
inserting a loose plug of cotton-wool in the top of the tube, 
and then suspending it in a large vessel full of sulphuretted 
hydrogen. A deposit of mercury sulphide will slowly form 
around the top of the tube.] 

Mercury Bichloride. 

Formula, HgClg: Combining weight, 271o 

Bichloride of mercury, or mercuric chloride, is familiarly 
known as corrosive sublimate. It is usually prepared by 
heating a mixture of mercury sulphate and common salt. Its 
colorless crystals are soluble in fifteen parts of cold or two 
of hot water. The addition of a little ammonium chloride to 
the cold water increases its power to dissolve the mercury salt. 
It is soluble in alcohol and in ether, and is a violent poison. 

In photography, corrosive sublimate is largely used for 
intensifying. The thin negative is soaked in a saturated solu- 
tion until it turns white (owing to the formation of calomel 


118 


THE CHEMISTRY OF PHOTOGRAPHY. 


(HggClg) and silver chloride), and then in a weak solution of 
ammonia until black. 

Mercury Sub-chloride. 

Formula, HggClg: Combining weight, 471. 

Mercury sub-chloride, or mercurous chloride, is the calomel 
of druggists. It is prepared by heating mercury with 
mercury bichloride. It is insoluble in water, alcohol, and cold 
dilute acids. When exposed to light it turns gray, owing to 
the separation of metallic mercury. 

Mercury Iodide. 

Formula, Hglg* Combining weight, 454. 

Mercuric iodide is formed when solutions of potassium 
iodide and of mercuric chloride are mixed together. It 
appears first as a yellow precipitate, but this rapidly changes 
to scarlet. It is soluble in excess of either of the solutions 
from which it is formed, more especially in excess of potas- 
sium iodide. 

Mercuric iodide, followed by ammonia, forms an excellent 
inteusifier for gelatine negatives. 

There is also a mercurous iodide^ Hgglg, which is formed 
by mixing solutions of potassium iodide and mercurous 
nitrate. It is of a greenish -yellow color. 

Mercury Mon-oxide. 

Formula, HgO: Combining weight, 216. 

Mercury mon-oxide is also called mercuric oxide, red oxide 
of mercury, or red precipitate. It can be obtained by heating 
mercury to a temperature rather below its boiling point for 
several weeks in a glass flask with a long neck. Commer- 
cially, it is prepared by heating a mixture of mercury and 
mercuric nitrate. It is usually seen as a bright red crystalline 
powder, but it can be obtained of an orange-yellow hue by 
adding caustic soda to a solution of a mercuric salt. Mercury 
mouroxide is a poisonous substance, slightly soluble in water. 


CHEMICALS EMPLOYED IN PHOTOGKAPHY. 


119 


When heated it darkens, but resumes its original tint on cool- 
ing. By strong heat, it is broken up into mercury and 
oxygen. 

Naphtha. 

True naphtha is a hydro-carbon which occurs naturally as 
mineral naphtha'’’’ in the rocks of Pennsylvania and Canada, 
and less abundantly in certain parts of Europe and Asia. 

Coal naphtha is a nearly identical substance, obtained by 
distillation from coal during the manufacture of coal-gas. 
Naphtha is a clear, limpid, oily liquid, which burns wdth a 
bright, smoky flame. It will not mix with water, but is a 
good solvent for caoutchouc (india-rubber). 

Owing to its freedom from oxygen, it is used to protect the 
metals sodium and potassium from the air, the bottles in 
which they are preserved being kept full of naphtha. 

The term “ wood naphtha,” or “ vegetable naphtha,” is 
sometimes applied to “ wood spirit” (methyl alcohol), but this 
is a misapplication, as the latter is a very different sub- 
stance. 

Nitric Acid. 

Formula, UNO 3: Combining weight, 03 . 

Nitric acid — often called aqua fortis — is prepared by distil- 
ling potassium nitrate with strong sulphuric acid. 

Commercial nitric acid has a yellow color, owing to the 
presence of nitric peroxide ; the pure acid is colorless. The 
yellow color can be destroyed by blowing air through the 
acid. It is a very corrosive liquid, producing dangerous 
wounds if it comes in contact with the skin. The dilute acid 
colors the skin, nails, clothes, etc., of a bright yellow color. 
Nitric acid is a very powerful oxidizing substance — that is, it 
readily parts with some of its oxygen to other bodies. It 
attacks all ordinary metals, except gold and platinum, forming 
a series of salts called nitrates, which are soluble in water. 

Nitric acid fumes strongly when exposed to the air, and has 
an irritating odor. It can be distinguished from other acids 
by the red fumes which are given off when it is poured on 
copper. 


120 


THE CHEMISTRY OF PHOTOGRAPHY. 


N itro-Hydrochloric Acid — A qua-Kegia. 

IS’eitlier nitric nor hydrochloric acid alone is able to dis- 
solve gold or platinnm. Yet a mixture of these acids — to 
which the name of aqua-regia has been given — readily 
dissolves either of these noble” metals. The reason is that 
by the mixing of the acids chlorine is set free, and this nascent 
chlorine unites with the metals to form chlorides, which are 
soluble. 

The mixture should be made in the proportion of one of 
nitric to three of hydrochloric acid, and, to lessen the violence 
of the action, an equal quantity of water may be added. 

Nitrous Acid. 

Formula, HNOg: Combining weight, 47. 

This is a very unstable substance, prepared by passing 
nitrous anhydride into water. Sometimes it acts as a reducing 
agent, precipitating gold and mercury from solutions; at 
others it exhibits oxidizing properties, liberating oxygen and 
becoming reduced to nitric oxide and water. Nitrous acid 
forms a series of salts called nitrites^ which behave similarly 
to the acid, but are much more stable. These nitrites can be 
distinguished from nitrates by the reddish fumes they evolve 
when treated with dilute acids. 

Oil of Lavender. 

Oil of lavender — an inferior variety of which is sold as “ oil 
of spike” — is made by distilling lavender flowers with water. 
It is a yellowish liquid, soluble in alcohol, but insoluble in water. 
In photography it has been used for dissolving bitumen and 
pyroxyline, for scenting certain pastes or cerates used in bur- 
nishing, and in varnishes. 

Oxalic Acid. 

Formula, 0211204 + 21120: Combining weight, 

90 + 36 =: 126. 

Oxalic acid, combined more especially with potassium, 
occurs plentifully in the vegetable kingdom, as in the leaves 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


121 


of the wood-sorrel, the stalks of rhubarb, etc. It is how made 
in large quantities by the action of caustic potash on sawdust ; 
but for experimental purposes, a small quantity is best pre- 
pared by acting upon sugar or starch with nitric acid. 

Oxalic acid is not very soluble in cold water, but more 
so in warm water and in alcohol. The solution is very 
poisonous, and, as the crystals are much like those of Epsom 
salts, it has been the cause of many accidents. The best 
remedy is the administration of powdered chalk suspended in 
water. 

Oxalic acid is much used in calico printing, and for taking 
ink stains out of linen. It is also employed for cleaning brass 
and leather. 

\Yhen crystallized, it forms prisms, whose composition is 
CgHgO^ + 2 II 2 O. By heating to 212 deg. Fahr., the water of 
crystallization is driven off, and a white powder remains. 
Oxalic acid forms two classes of salts called normal or alkaline 
oxalates, and acid oxalates. The former are all soluble, the 
latter generally insoluble, in water. 

Ozone. 

Symbol, O 3 : Molecular weight, 48. 

In 1840, Schbnbein showed that ozone is an allotropic 
form of oxygen. Each molecule of ordinary oxygen contains 
two atoms, while in the molecule of ozone three atoms are 
crowded together, so that any volume of ozone weighs half as 
much again as the same volume of oxygen. 

Ozone is now usually produced by submitting oxygen to the 
silent electrical discharge. It may be detected by the blue 
coloration which it produces in paper that has been dipped 
first into starch paste and then into potassium iodide solution. 

Ozone is a very powerful oxidizing agent, releasing readily 
its third atom of oxygen. Thus both silver and mercury, 
upon which oxygen has little or no effect, are quickly tar- 
nished by ozone. 

Holmes’ ozone bleach is a substance sold commercially (it is 
an alkaline hypochlorite), which is an effeetive reducer for 
over-dense negatives. 


122 


THE CHEMISTEY OF PHOTOGEAPHY. 


Palladium and its Compounds. 

Palladium is a metal often found associated with platinum. 
Its formula is Pd, and atomic weight 107. Like platinum it 
forms two series of chlorides, PdClg and PdCl 4 . Palladium 
bichloride, PdClg is a dark-brown powder which forms double 
salts bj combining with alkaline chlorides in a similar manner 
to the corresponding platinum compounds. Of these double 
salts the chloro-palladite of potassium (PdCl 2 , 2 KCl) has been 
used in photography for toning prints, transparencies, and 
enamels. 

Paea-amidophenol. 

Formula, CgIl4l7H2 0H : Combining weight, 109. 

Many organic compounds possess the property of reducing 
salts of silver to the metallic state. But as photographic 
developers only those are of use which will reduce salts of 
silver that have heen exposed to lights while leaving intact non- 
exposed salts. The latter class are comparatively limited in 
number ; but to Dr. Andresen, of Berlin, we owe an addition 
to this class in the form of the substance known as para-amido- 
phenol, which is chemically related to hydrochinone and to 
eikonogen. It was introduced by him in 1891 as a brownish 
crystalline powder, slightly soluble in cold water, more so in 
hot water. 

But by the aid of acids, salts are obtained from para-amido- 
phenol which are easily soluble in water. As an example we 
have hydrochloric para-amidophenol — CgH 4 NIl 2 HC 10 II — 
which, combined with carbonate of potash, makes a good 
developer. 

By the action of caustic soda we get para-amidophenol- 
natrium — Cg Il 4 NIl 20 ]Sra ; and by the action of caustic pot- 
ash we obtain para amidophenol-potassium — CgH 4 lSril 20 K ; 
these two alkaline compounds are easily dissolved even by 
cold water. 

Kodinal is the name given to a concentrated solution of 
one of the above forms of para-amidophenol. It is a reddish- 
brown liquid which requires diluting with from fifty parts 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


123 


(for negatives) to one hundred parts (for bromide paper) of 
water for use. 

Phosphoric Acids. 

There are three distinct substances, to each of which the 
term phosphoric acid ” has been more or less frequently 
applied. 

Meta- Phosphoric Acid^ IIPO 3 , combining weight 80, i& 
produced when phosphoric anhydride (PgOg), the white 
powder produced by burning phosphorus in oxygen, is dis- 
solved in cold water. 

Phosphoric Acid^ II3PO4, combining weight 98, is best 
obtained by distilling nitric acid with amorphous phosphorus. 
It is used in photography in Willis’ aniline process for the 
copying of plans. 

Phosphoric acid forms a series of salts called phosphates, 
which are distinguished by the yellow precipitate they give 
with solutions of silver nitrate. 

Pyro- Phosphoric Acid^ II 4 P 2 O,, combining weight 178, 
is obtained — as the name implies — by heating phosphoric acid 
until water is driven off. 

Phosphoric (or ORrHo-piiosPHORic) Acid. 

Formula, II 3 PO 4 : Combining weight, 98. 

This acid can be prepared by heating red phosphorus in a 
retort with common strong nitric acid. On a large scale it is 
made by dissolving bone-ash in sulphuric or hydrochloric acid. 
In commercial phosphoric acid, arsenic acid is frequently 
present as an impurity. 

Pure phosphoric acid forms colorless crystals, which are 
very soluble in water. The solution tastes intensely 
sour, and reddens blue litmus. It is not poisonous. The 
best test for phosphoric acid is molybdate of ammonia, an 
acid solution of which is turned yellow by phosphoric acid. 
When heated to a dull red in a platinum crucible, phosphoric 
acid is converted into a transparent mass of meta-phosphoric 
acid — HPO 3 ; this is the glacial phosphoric acid of druggists. 


124 


THE CHEMISTRY OF PHOTOGRAPHY. 


By long continued lieat phosphoric acid may be changed 
finally into another modification, called pyro phosphoric 
acid— n^p.o,. 

A solution of the ordinary, or ortho-phosphoric, acid is used 
in Willis' aniline process. Dr. Maddox finds that the addition 
of a trace of phosphoric acid to the ordinary pyro-ammonia 
developer, improves the color of the image and tends to pre- 
vent fog. 

Photo-Salts. 

During the year 188Y, Mr. Carey Lea, of Philadelphia, pub- 
lished, in the American Journal of Science^ the results of a 
long series of researches upon the nature of the change effec- 
ted by light upon the haloid salts of silver. Previously, Mr. Lea 
had been the principal advocate of the theory which states 
that the first effect produced by light is simply a physical 
change, predisposing the elements of the silver haloid to 
dissociation, so that when a reducing agent (the developer) is 
applied, the molecules so affected yield more quickly to its in- 
flnence.” 

The other, or chemical theory of development, declared that 
the effect of light was to remove some of the haloid element — 
the chlorine, bromine, etc. — combined with the silver, leaving 
a sub-salt, which was readily reduced by the developer: 

2AgCl = AgsCl + Cl 

Silver yields Silver and Chlorine. 

Chloride Sub-chluride 

Mr. Lea’s later researches have led him to believe in a modi- 
fication of this chemical theory. 11 e finds that light decomposes 
a small part of the silver salt, and that the sub-salt then forms 
a molecular combination with the unaltered salt. To such a 
molecular combination Lea applies the name of a photo-salt,” 
and speaks of photochloride of silver,” or “ photobromide of 
silver,” as the the case may be. The proportion of the sub- 
salt” in the combination may vary from a very minute 
quantity up to eight or nine per cent. These photo-salts 
exhibit a wide range of coloration, from white through pink, 
and purple to black. The typical photochloride of .silver is of 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


125 


a magnificent red hue. It is possible that the way to “ photog- 
raphy in colors” lies through these photo-salts. A very 
important part of Mr. Lea’s discovery lies in the fact that he 
has been able to prepare these photo-salts chemically, without 
the action of light. 

Platinum. 

Symbol, Pt : Combining weight, 194. 

Platinum is found only in the metallic state. Grains and 
nuggets of this metal occur in the sands of rivers in the Ural 
Mountains, Borneo, California, etc. It is a wUite, very malle- 
able and ductile metal, which never tarnishes, since platinum 
does not combine directly with oxygen at any temperature. 

No single acid can dissolve platinum, but aqua-regia, or any 
liquid capable of evolving chlorine will attack it. The high 
fusing-point of platinum — about 4,000 deg. Fahr. — and its 
power of resisting chemical action, specially fit it for use in 
the chemical laboratory, and render it serviceable to the photog- 
rapher. Platinum crucibles, basins, spatulas, foil, and wire 
are frequently required. Platinum crucibles should never be 
put naked into a coke or charcoal fire, but always placed 
within a covered earthen crucible. They should never be 
used for melting any of the oxides of a readily fusible metal, 
such as lead or tin, as these metals will combine with the 
platinum and form an alloy, and the vessel will be destroyed. 

Platinum Tetrachloride (Platinic Chloride). 

Formula, PtCl 4 + 5HoO : Combining weight, 

33G + 90 = 426. 

Prepared by dissolving platinum in aqua-regia and evapo- 
rating several times, each time adding hydrochloric acid. A 
compound having the formula PtCl 4 , 2HC1 is thus obtained, 
from which the hydrochloric acid may be expelled by heat. 
Platinic chloride forms red crystals, which are soluble in 
water, producing an orange-colored solution. When strongly 
heated, platinic chloride parts with two atoms of chlorine and 
is reduced to platinoiis chloride, PtCl 3 . Platinic chloride 
combines with other chlorides, espei^ially those of the alkali 


126 


THE CHEMISTRY OF PHOTOGRAPHY. 


metals, to form a series of double chlorides, which vary greatly 
in their solubility. Potassium platinic chloride (PtCl 4 , 2KC1), 
for example, is insoluble in water, while sodium platinic 
chloride (PtCl 4 , 2 hTaCl), is readily soluble. The former salt 
— more generally known, perhaps, as potassium chloroplatinite 
— is largely used in the platinotype process. 

PoTASSIO FERRIC OxALATE. 

Formula, FcgCC 304 ) 3 , 3 K 3 C 2 O 4 : Combining weight, 8Y4. 

This constitutes the green crystals seen at the bottom of a 
hot-bath platinotype solution after it has been used several 
times ; it is also found in the old ferrous oxalate developer. 
It is soluble in water ; insoluble in alcohol. It is decomposed 
by light, and hence is used in certain blue ” processes. 

Potassium. 

Symbol, K : Atomic weight, 39. 

Metallic potassium was first obtained by Davy, in 1807. 
He decomposed caustic potash by a strong current of electric- 
ity, and obtained a silvery-white, soft metal, which tarnished 
instantly on exposure to air, owing to its great affinity for 
oxygen. For this reason potassium is usually kept under some 
liquid which contains no oxygen, as petroleum. Potassium 
decomposes water at all temperatures, forming potassium 
hydrate (caustic potash), and liberating hydrogen, the energy 
of the chemical combination being sufficient to infiame the 
escaping hydrogen. 

Potassium Bichromate. 

Formula, KgCrgO^: Combining weight, 294. 

The chromium compounds are obtained by heating chrome 
iron *ore with potash carbonate, by which means a soluble 
yellow chromate, K 2 Cr 04 , is formed. When sulphuric acid 
sufficient to combine with half the potassium in the yellow salt 
is added, the Jjichromate of potash is formed, and crystallizes 
out in large red crystals as the solution cools. Potassium 
bichromate dissolves in ten parts of cold water, but is much 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


127 


more soluble in hot water. It is insoluble in alcohol, and is 
very poisonous. 

When bichromate of potash is mixed with an organic sub- 
stance, such as gelatine, and exposed to light, it becomes 
dark-colored, owing to the liberation of oxygen, and the con- 
se(-|uent reduction of the bichromate to chromic oxide, CrgOg. 
A further effect is that the gelatine is rendered insoluble in 
water, and non-absorbent. Advantage is taken of this in the 
carbon printing process in photography, powdered carbon 
being mixed with the bichromatized gelatine, which is then 
exposed to light beneath a negative, and finally washed in hot 
water. The portions unacted on by light are dissolved away, 
while the insoluble parts remain to form the picture. Potas- 
sium bichromate has a very injurious effect upon the skin if 
there be any cuts or scratches through which it can enter. 

Potassium Bromide. 

Formula, KBr: Combining weight, 119. 

Prepared by dissolving bromine in caustic potash, whereby 
a mixture of bromide and bromate of potash is produced. 
This is evaporated down to dryness, and gently ignited to 
drive off the oxygen, by which the bromate is reduced to 
bromide also. Potassium bromide forms clear cubical crystals, 
which are readily soluble in water, slightly soluble in alcohol. 
It is a favorite restraining agent in the ordinary pyro 
developer, preventing any action upon the silver bromide 
which has not been affected by light, and steadying and regu- 
lating the decomposition of that which has. 

Potassium Carbonate. 

Formula, Kg CO 3 : Combining weight, 138. 

Carbonate of potassium was formerly known as salts of tartar, 

potashes,” or pearl-ash.” The original source of this potas- 
sium salt was the ashes which resulted from the burning of wood 
or other vegetable matter. When such ashes were boiled in pots 
the carbonate of potassium was extracted from them, and it was 
then easily obtained in the solid s»ta*te by evaporating the water. 


128 


THE CHEMISTRY OF PHOTOGRAPHY. 


Of late years much has been obtained from beet-root, and from 
the potassium sulphate, which occurs in such vast deposits at 
Stassfurt in Germany. 

The pearl-ash ” of commerce contains small quantities of 
sodium carbonate, and potassium sulphate, etc. Potassium 
carbonate is a white deliquescent substance, very soluble in 
v^rater, but insoluble in alcohol. Owing to its affinity for 
water, it is employed in removing the last traces of water from 
alcohol. It is a strongly alkaline salt. 

“ Potash ” — as Kg CO 3 is familiarly called — is largely used 
to render alkaline the pyro developer. It must be carefully 
distinguished from the acid potassium bicarbonate (bicarbonate 
of potash), KHCO 3 . 

Potassium Chlorate. 

Formula, KCIO 3 : Combining weight, 1221 . 

Chlorate of potassium can be made by passing chlorine into 
a strong solution of caustic potash or of potassium carbonate. 
It is now largely manufactured by passing chlorine into milk 
of lime and then adding potassium chloride. 

Chlorate of potassium forms flat, shining crystals having an 
acid and cooling taste, like nitre. 

When heated to about 670 deg. Fahr., these crystals decom- 
pose and all the oxygen is liberated ; hence this salt is largely 
used as a source of oxygen gas. By mixing with the chlorate 
of potassium one-fifth of its weight of black oxide of manganese, 
the heat required to liberate the oxygen is greatly reduced. 
One pound of the salt should produce four cubic feet of oxygen. 
As the commercial chlorate always contains small quantities of 
chlorine, the oxygen gas should be purified by passing it 
through two wash-bottles partly filled wdth water, in which a 
little caustic potash or potassium carbonate has been dissolved, 
before it is allowed to enter the bag in which it is to be stored. 

Potassium Chloride. 

Formula, KCl : ' Combining weight, 74 ^. 

Chloride of potassium is much like rock-salt in appearance and 
properties. Thick beds of potash-salts occur near Stassfurt, 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


129 


probably formed by the drying up of some old lake or inland 
sea. It forms colorless cubes, which are soluble in three parts 
of cold water ; more soluble in hot water, but insoluble in 
alcohol. 

Potassium Citrate. 

Formula, KgHgCgO^ : Combining weight, 306. 

Made by adding carbonate of potassium to a solution of citric 
acid until the latter is neutral ; then evaporate to dryness. 
Citrate of potassium is a white crystalline powder which deli- 
quesces when exposed to the air. It is very soluble in water ; 
insoluble in alcohol. It acts as a preservative of ordinary sen- 
sitized paper ; being either mixed with the silver bath or the 
paper may be floated upon it after silvering. It also acts as a 
powerful restrainer in alkaline development. 

Potassium Cyanide. 

Formula, KCH (or KCy) : Combining weight, 65. 

Prepared by fusing dry ferrocyanide of potassium with 
potassium carbonate in an iron crucible. 

The* iron separates and sinks to the bottom, when the 
liquid potassium cyanide can be poured ofl, and, being allowed 
to cool, solidifies to a white cake, which can be broken up for 
use. Owing to imperfect mixture or fusion, potassium carbon- 
ate is frequently present, as an impurity, in the commercial 
salt ; but its presence is not directly harmful. 

Cyanide of potassium emits an odor of prussic acid, and 
gives off that substance freely when any acid is added to it ; 
it is highly poisonous. The aqueous solution dissolves gold 
and silver, forming double cyanides, which are largely used 
for electro-gilding and electro-plating. Potassium cyanide 
was largely used as a flxing agent , during the wet-collodion 
epoch’’ of photography, but for gelatine plates it has been 
displaced by hyposulphite of soda. 

Potassium Ferricyanide (Ped Prussiate of Potash). 

Formula, KgFeCyg: Combining weight, 329. 

Prepared by passing chlorine gas through a solution of 


130 


THE CHEMISIEY OF PHOTOGKAPHY. 


potassium ferrocyanide ; the latter loses one atom of potas- 
sium and is converted into the ferricyanide. This salt forms 
beautiful red crystals, which are soluble in two and one-half 
parts of cold, or one and one-half of boiling water ; insoluble 
in alcohol. 

Potassium Ferrocyanide (Yellow Prussiate of Potash). 

Formula, K4FeCyg+3H20 : Combining weight, 

368 + 54 = 422. 

Prepared commercially by heating nitrogenous organic 
matter — as horn.; hide-parings, etc. — with potashes and iron 
filings. The fused mass is heated with water, which, on 
evaporation, then yields tough yellow crystals. It is soluble 
in four parts of cold, or two parts of hot water ; insoluble in 
alcohol. 

Potassium Fluoride. 

Formula, KF : Combining weight, 58. 

Fluoride of potassium is made by neutralizing hydrofiuoric 
acid with caustic potash. The cubical crystals have a saline 
taste and deliquesce in air. 

Potassium Hydrate (Caustic Potash). 

Formula, KHO : Combining weight, 56. 

Caustic potash — or ‘‘potash,” as it is sometimes termed — is 
formed when metallic potassium is placed in water. It is 
usually prepared by adding slacked lime (calcium hydrate) to 
a rather weak hot solution of potassium carbonate. Chalk is 
formed, which sinks to the bottom, and the clear liquid is 
decanted and evaporated to dryness, when the caustic potash 
remains as a hard, white, brittle solid. Lastly, it is fused, and 
cast into sticks, in which state it is usually sold. 

Potassium hydrate is a powerful alkali, burning the skin, 
and neutralizing acids. It is largely used in soap-making. 
Since it is very- deliquescent the sticks should be kept in a 
stoppered bottle. Caustic potash dissolves in about half its 
weight of water. Caustic potash works admirably with hydro- 
chinon as a developer for gelatine dry plates. 


CHEMICALS EMPLOYED IN PHOTOGRAPHY'. 


131 


Potassium Iodide. 

Formula, KI : Combining weight, 166. 

Prepared by digesting iodine with water and iron filings, 
and then adding potassium carbonate. It crystallizes in cubes 
which are very soluble in water, slightly soluble in alcohol. 
The pure salt should be neutral, but, as usually met with, it 
has an alkaline reaction. The aqueous solution dissolves 
iodine freely. 

Potassium Meta-Bisulphite. 

Formula, Kg SO 3 , SO^: Combining weight, 

This salt may be obtained by passing suljihurous anhydride 
in excess into a solution of potassium carbonate, and adding 
alcohol. Care must be taken to keep the sulphurous 
anhydride in excess, or else the normal sulphite will be 
formed. The meta-bisulphite of potash w'as introduced in 
1887 by Messrs. Mawson & Swan as a preservative of pyro 
when in solution. 

Potassium Nitrate. 

Formula, KNOgi Combining weight, 101 . 

Nitrate of potassium, familiarly known as “nitre,” or “salt- 
petre,” forms a surface-deposit on the soil of many hot 
countries, as Bengal, Egypt, etc. It is also prepared by mix- 
ing solutions of sodium nitrate and potassium chloride. 

Potassium nitrate is soluble in five parts of cold, and in its 
own weight of hot water. It contains nearly half its weight 
of oxygen, with which it readily parts when heated with any 
combustible substance. For this reason nitre is much used in 
the manufacture of gunpowder and fireworks. 

Potassium chloride is frequently present in ordinary nitre. 
Its presence may be detected l)y the white precipitate 
produced by the addition of a few drops of silver nitrate. 

Potassium Nitrite. 

Formula, KNOg! Combining w'eight, 85. 

Potassium nitrite can be produced by heating the nitrate 


132 


THE CHEMISTRY OF PHOTOGRAPHY. 


until of its oxygen is driven off. This decomposition 
takes place more readily when some oxidizahle metal, such as 
lead, is present. 

]^i trite of potash forms small, white crystals, which 
deliquesce in air, and are insoluble in absolute alcohol. 

The use, in photography, of KNO 3 depends mainly on the 
fact that it is a halogen absorbent. Bromide paper, treated 
with a solution of potash nitrite, forms an excellent actino- 
meter. The paper should be soaked for fen minutes in a ten 
per cent, solution, and allowed to dry slowly in the dark. In 
strong sunlight, such paper will attain its deepest color- 
indigo blue — in twenty-five seconds. 

Potassium Oxalate. 

F ormula, K 3 C 3 O 4 + 2 H 3 O: Combining weighty 

176 + 36 == 212. 

The neutral oxalate of potash (which is the salt employed 
by photographers) is prepared by neutralizing oxalic acid with 
potassium carbonate. It crystallizes in transparent prisms, 
which dissolve in three parts of water. When heated, the 
crystals part with their water of crystallization and become 
white and opaque. The binoxalate, or acid oxalate of potash, 
can be distinguished by the sour taste of its crystals ; its 
formula is C 3 HKO 4 . It is also known as salt of sorrel^ from 
its occurrence in that plant. 

Tlie neutral potassium oxalate is employed in the prepara- 
tion of ferrous oxalate, which is largely used as a developer 
for paper negatives and transparencies, and — chiefly on the 
Continent — for gelatine dry-plates also. It is also much used 
in the platinotype printing process. 

Potassium Permanganate. 

Formula, KMnO^: Combining weight, 158. 

Permanganate of potash is made by pouring boiling water 
on potassium manganate, and then filtering through asbestos or 
glass wool. Its prismatic crystals are red by transmitted, but 
black by reflected light. It is soluble in sixteen parts of 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


133 


water, and the solution — sold as ‘‘Condj’s Fluid” — is a well- 
known disinfectant. Potassium permanganate is a useful 
oxidizing agent. 

Potassium Sulphate. 

Formula, KgSO^: Combining weight, 1T4. 

Potassium sulpliate is largely produced as a bye-product, in 
the manufacture of bichromate of potash and certain other 
substances. It forms colorless crystals, which dissolve in ten 
jiarts of cold or four of boiling water. 

Potassium Silver Cyanide. 

Formula, KAg(CN) 3 : Combining weight, 200. 

This ^bstance crystalhzes in feathery tufts or hexagonal 
prisms. It is soluble in four parts of water, and is unaffected 
by light. 

Potassium Sulphide. 

Formula, KgS: Combining weight, 110. 

Potassium and sulphur combine in several ^proportions, of 
which the mono-sulphide, KgS, is perhaps the best known. 
It can be made by dividing a saturated solution of caustic 
potash into two parts, passing sulphuretted hydrogen through 
one part and then adding the other half. It is an alkaline, 
caustic body. 

Potassium Sulpho-cyanide. 

Formula, KS(C]N^): Combining weight, 97. 

Prepared by heating yellow prussiate of potash with carbon- 
ate of potash and sulphur, and boiling the mass with alcohol. 
It is a transparent, crystalline substance, very soluble in water. 
"When five parts of the salt are dissolved in four parts (by 
weight) of water, a temperature of — I deg. Fahr. is pro- 
duced. Sulpho-cyanide of potassium has been used as a 
fixing agent, especially for positive pictures, in place of 
hyposulphite of soda. It is present in human saliva, a fact 
which may affect the permanency of jihotographs that have 


134 


THE CHEMISTRY OF PHOTOGRAPHY. 


had the tongue passed over them (a common practice) in 
order to induce the glossy surface to take tints or colors more 
readily. It is also used in the toning-hath for gelatino-chloride 
and collodio-chloride prints. 

Primuline. 

Primuline is the “ trivial ” name of a yellow dye obtained 
by the action of sulphur upon toluidine, a coal-tar base closely 
allied to aniline. It was discovered by Mr. A. G. Green in 
1887, and was first used as a dye for calico, one of its peculiar- 
ities being that it requires no mordant. Its photographic 
properties were announced at the British Association meeting 
at Leeds in 1890. The chemical composition of primuline is 
very complex and is perhaps hardly known with certainty. 
Primuline itself is not sensitive to light, but by treatment with 
nitrous acid it is converted into diazo-primuline which, when 
in contact with cotton, linen, paper, etc., is rapidly aflPected by 
exposure to the sun. Further, the diazo-primuline has the 
power of combining with phenols or amines to form brightly 
colored compounds ; but after exposure to light it loses this 
power. It is therefore possible to secure colored prints upon 
calico, etc., in the following manner : (1) Soak the calico in a 
solution of primuline, then rinse and soak in a solution of 
nitrous acid. (2) Dry the calico and expose it to light beneath 
a positive. (3) Develop by soaking the exposed calico in one 
of various solutions according to the color desired, as resorcin 
(orange), betanaphthol (red), phenol (yellow), etc. ; finally, rinse 
and dry. The process gives a positive from a positive ; or a 
negative picture from a negative. 

Prussian Blue; FE4(FECYg)3. 

There are several varieties of this useful substance, which 
is largely employed in painting. When a ferric salt is added 
to potassium ferrocyanide a blue jDrecipitate of soluble Prus- 
sian bhie^ Fe 4 K 3 Cyi 2 , is produced. This substance dissolves 
in pure water, but is insoluble in saline solutions. By adding 
ferric chloride to a solution of soluble Prussian blue a deep 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


185 


blue powder is precipitated, wliicb is insoluble Prussian hl/ue^ 
Fe^Cjis, and this is the ordinary, or commercial article. It 
is sold in cubical dark-blue lumps, and is insoluble in water 
and in weak acids. It is soluble in oxalic acid, forming a dark- 
blue liquid, which is used as an ink. 

Pyrocatechen. 

Formula, C 6 H 4 (OH) 2 . Combining weight, 110. 

Also known as catechol, and as brenzcatechin. Has the 
same relative composition as hydroquinone and resorcin ; but 
tlie atoms in the molecule are differently arranged. This sub- 
stance is sold in whitish crystals, which are soluble in water 
and in alcohol. It forms a fairly good substitute for pyro- 
gallol in the alkaline development of dry plates. 

Pyrogallic Acid (Pyrogallol). 

Formula, CgHgOgi Combining weight, 126. 

Pyrogallic acid — as the name implies— is prepared from 
gallic acid by the action of heat. The gallic acid may be 
placed in a porcelain crucible, over the top of which a piece 
of blotting-paper is then tied, the whole being covered and 
surmounted by a paper cone. AYith a Bunsen burner, cr 
spirit-lamp, the temperature is tlien raised to 350 deg., when 
the gallic acid is decomposed into pyrogallic acid — which rises 
through the pores of the blotting-paper and settles on the in- 
side of the paper cap — and carbonic acid gas, which escapes. 
The great drawback to this, and indeed to most methods of 
preparing the substance, is that a large part of the gallic acid 
is decomposed into metagallic acid, Cgll^O^, so that only 
about one-fifth of the gallic acid is converted into pyrogallic 
acid. 

An improvement introduced by Liebig is to mix powdered 
pumice with the gallic acid, and pass a slow stream of carbonic 
acid gas over the mixture so as to remove the pyrogallic acid 
before it has had time to become over-heated. By this method 
the yield is nearly doubled, but is still less than half the pos- 
sible amount. For an experiment on a small scale the best 


136 


THE CHEMISTRY OF PHOTOGRAPHY. 


method is that devised by Professor Thorpe, of heating gallic 
acid in glycerine (150 grains to each ounce) in a glass retort. 
The temperatnre of the liqnid must not rise above 400 deg. 
Fahr. The heat drives off carbonic acid gas, and a solution 
of pyrogallic acid in glycerine is left behind, which will 
keep ” for months. For preparing ‘‘ pyro ” on a large scale, 
an aqueous solution of gallic acid is heated to 400 deg. Fahr. 
in a closed vessel for thirty minutes. The solution is then 
boiled with animal charcoal, filtered and evaporated to dry- 
ness. The solid residue so obtained is then distilled by gently 
heating it in a vacuum. In this way nearly all the gallic is 
converted into pyrogallic acid. 

Pyrogallic acid has not the characteristic properties of an 
acid — it has a bitter, not a sour taste; and it does not redden 
blue litmus — hence chemists do not consider it a true acid, 
and in chemical text-books it is now termed pyrogallol,” but 
it is familiarly known to photographers as pyro.” 

Pyrogallol forms brilliant crystalline plates, which break up 
into a fine feathery powder, so light as to be scattered by a 
breath. It is extremely soluble in water, alcohol and ether. 
It melts at 239 deg. Fahr., and when the liquid boils it gives 
off a colorless, irritating vapor. Aqueous solutions of pyro 
abstract oxygen from the atmosphere, and from the air dis- 
solved in ordinary water, quickly turning brown and becoming 
useless to the photographer. The addition of a little citric or 
nitric acid retards this change. A solution of j)yro in glycerine 
and alcohol keeps fairly well. When the solution of pyro is 
rendered alkaline, it becomes first yellow and then brown, a fact 
which distinguishes it from gallic acid, which undergoes no 
such changes. With solutions of pure ferrous salts pyrogallol 
gives a fine blue tint, which the least trace of a ferric salt 
changes to green. 

Pyro is an active reducing agent, absorbing oxygen so 
eagerly that it decomposes most of the salts of the ‘‘noble 
metals” — gold, silver, and platinum. For this reason it has 
been in constant use in photography for the last forty years ; 
and its price has been reduced as the demand for it became 
greater, from 10^. or 15^. per ounce, to a shilling or even less. 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


137 


Owing to its power of absorbing oxygen, pyrogallic acid is 
always used for that purpose in gas analysis. 

Pyroxyline. 

Formula, Cj gH22(N02)g0^ 5 : Combining weight, 81 : 6 . 

When cotton wool is steeped in a mixture of equal parts of 
strong nitric and sulphuric acids the formidable explosive 
known as gun-cotton (C^ gllg i(N02)90^ g) is produced, which 
is quite insoluble in any mixture of alcohol and ether. But 
when the acids are mixed in the proportion of three parts of 
sulphuric to one of nitric, and a certain quantity of water — say 
seven-eighths of a part — is added, then the cotton which is im- 
mersed in the mixture acquires different properties. It is not, 
explosive, and it is soluble in a mixture of alcohol and ether. 
Either cotton-wool, straw, paper, pitch, flax (as linen) or cal- 
ico, etc., may be used, but in each case the resulting product 
will be slightly different. When paper is used the resulting 
product is known papyroxiline. The chemical composition 
of each of these substances is CglT^oOg, which may be 
regarded as six atoms of carbon combined with five molecules 
of water (H^ ^Og = 5II2O). The sulphuric acid combines 
with the water, which is invariably present in even the purest 
nitric acid, and the anhydrous nitric acid is then able to 
attack the cotton, displacing either two or three of its atoms of 
hydrogen and replacing them by an equal number of nitrosyl 
(NO 2) groups. The presence of nitrogen tetroxide in the 
altered cotton may be proved by the red fumes which are 
seen when the cotton is ignited m a glass globe exhausted of 
air. 

Potassium nitrate (KNO3) may be used instead of nitric 
acid, the latter being then formed by the action of the sul- 
phuric acid on the potassium nitrate. 

Kesorcin. 

Formula, CgH4(OH)2 : Combining weight, 110. 

Prepared by a complex pi ^oess from benzole and sulphuric 
acid. Is soluble in water and in alcohol. Besorcin is isomeri 


138 


THE CHEMISTRY OF PHOTOGRAPHY. 


in composition with liydroquinone and with catechol ; and 
some experimenters say that it may be used similarly in the 
composition of an alkaline developer. But Dr. Andresen finds 
that piire resorcin has no effect upon silver bromide, and at- 
tributes its supposed developing powers to the fact that traces 
of its isomers were present in the samples used by those who 
noted its (supposed) developing powers. 

Salicylic Acid. 

Formula, C^HgOg : Combining weight, 138. 

Prepared by passing carbonic acid gas through a heated 
mixture of caustic soda and carbolic acid. Soluble in 700 
parts of cold, or 9 parts of boiling water, or in 4 of alcohol. 
Is a powerful germicide, and a few drops of the solution added 
to any mountant, or to solutions of alum or of citric acid, will 
prevent decomposition. 

Shellac. 

A resinous substance deposited by an insect upon the twigs 
of plants in India, etc. The crude “lac” as imported is 
known as “ stick-lac,” since it includes the jjieces of wood, etc., 
upon which the insect deposited the resin. The “lac” is 
freed from the wood by rolling, grinding and washing, and is 
then called “ seed-lac.” When the seed-lac is melted and cast 
into thin layers, it is called “shellac,” or “button-lac” if cast 
into sticks. By dissolving shellac in caustic soda, and passing 
chlorine through the solution, the natural brown or red color 
of the lac is removed, and we then get “bleached lac.” 

Shellac is soluble in alcohol, especially if a little oil of lav- 
ender be added. In photograiDhy it is much used as a var- 
nish for negatives ; but it should have about live ]3er cent, of 
sandarac added to it, in order to make it dry as a smooth level 
sheet. 

Silver. 

Symbol, Ag. (from argentum). Atomic weight, 103. Specific 
gravity, 10^. Melting point, 1900 degrees Fahrenheit. 

The alchemists called silver L^ma or Diana^ from its white 


CHEMICALS EMPLOYED m PHOTOGKAPHY. 


139 


color, like that of the moon. In nature, silver is found pure 
or native” in Peru, I^orway, etc., and its ores are not un- 
common. 

It is a very maileable and ductile metal, and is the best con- 
ductor known of heat and of electricity. Pure silver is too soft 
for use in the arts, so that it is usually alloyed with copper. 
The ‘^standard silver” of which our silver coins are made 
contains 92|- per cent, of silver and T-J per cent, of copper. 

The best solvent for silver is dilute nitric acid, but boiliug 
strong sulphuric acid will also dissolve it. 

In photography, silver was much used in the daguerreotype 
process, by which photographs were produced on thin plates of 
silver supported by a copper backing. The purity and cleanli- 
ness of the surface of the silver plate are of the highest im- 
portance in this process. 

Silver is not affected by pure air, oxygen or water ; but 
ozone and sulphuretted hydrogen cause it to tarnish. Silver 
hooks are frequently employed to raise the plates from the 
developing solution, and in the collodion process the plate 
rests, in tlie dark slide, upon silver wire. 

Silver, when melted, absorbs or occludes several times its 
volume of oxygen from the air. 

When the metal solidifies this oxygen is forced out, giv- 
ing the peculiar arborescent appearance often noticed on 
masses or buttons of pure silver, and which is known as the 
^‘silver tree.” During 1890-91 Mr. Carey Lea shoAved that 
silver could exist in several “ allotropic ” states, being then 
sometimes of a yellow, sometimes of a blue color, and possess- 
ing distinct properties. 

Silver Acetate. 

Formula, CgllgAgOg : Molecular weight, 167. 

Silver acetate is formed (1) by addition of silver nitrate to 
a strong solution of an acetate ; (2) by dissolving silver car- 
bonate in hot acetic acid. It is an exception to most of the 
acerates in that it requires 100 parts of water to dissolve one 
part of the salt. 


140 


THE CHEMISTRY OF PHOTOGRAPHY. 


.Silver acetate forms white flat crystals. Carbonate of silver 
is frequently present, as an impurity, in commercial silver 
acetate. 

Silver Ammonio-Nitrate. 

Formula, AgNOg + 2 NH 3 . Combining weight, 204. 

If ammonia is added to a neutral solution of silver nitrate 
until the precipitate produced is barely re-dissolved, and the 
solution then allowed to evaporate, fine bright prismatic crys- 
tals of ammonio-nitrate of silver will be produced. 

Plain salted paper may be advantageously sensitized with 
this salt, but it is unsuited for albumenized paper, as the am- 
monia dissolves the albumen. 

Silver Bromide. 

Formula, AgBr: Molecular weight, 188. 

Silver bromide is found native in very small quantities in 
Alexico, Chili, and Brittany. It may be prepared by the direct 
combination of its elements, as in the daguerreotype process, 
where a plate of silver is exjiosed to the vapor of bromine. In 
the collodion and gelatine processes of photography silver bro- 
mide is formed by the action of silver nitrate upon a soluble 
bromide, as : 

AgNOg + NH^Br = NH4NO3 + AgBr 

Silver (_ combines ( Ammonium to Ammonium | ajid J Silver 

Nitrate ( with / Bromide form Nitrate f | Bromide. 

When hydrobrornic acid is added to solutions of silver salts, 
silver bromide is precipitated. It is a yellowish-white sub- 
stance which alters to gray on exposure to light, a change 
which is retarded or altogether stopped by the presence of even 
a trace of nitric acid or free bromine. Silver bromide is in- 
soluble in water, but soluble in alkaline hyposulphites, 
cyanides, sulplio-cyanides, and in ammonia. 

The different modifications of silver bromide, which are 
sharply distinguished by their relative sensitiveness to light, 


CHEMICA:feS EMPLOYED IN PHOTOGRAPHY. 


141 


also present certain physical differences which are indicated 
in the following table : 


By Transmitted Light. 

ByReflected Light. 

1 

Occurrence. 

» 

r Orange. 

{ Slate blue. 

1 Bluish-white. 

In fresh collodion emulsion. 
Older bromide of silver in 
collodion wet-plates. 

Semi- 

transparent. 


Bluish-white. 

In very sensitive wet col- 
lodion plates. 


Reddish- 

orange. 

Yellowish-white. 

i 

In very old bromide of sil- 
ver in collodion. 

Almost 
opaque. j 

{ Violet- 
blue. 

r Yellowish-white. 
Greenish-yellow. 

Green, or violet-1 
green. | 

Very sensitive collodion 
emulsion. 

Bromide of silver in gela- 
tine ; sensitiveness me- 
dium. 

Very sensitive gelatine emul- 
sion. 

1 

Blue. 

Indistinct. 

Slightly sensitive silver 
bromide in collodion, 
yielding indistinct pic- 
tures. Affected by red 

end of spectrum. 


Silver Carbonate. 

Formula, AggCOg : Combining weight, 276. 

This is a yellowish-white powder formed by adding an 
alkaline carbonate to a solution of silver nitrate. It is solu- 
ble in ammonia and dilute acids ; slightly soluble in water. 

When exposed to light, or heated, silver carbonate darkens. 
It is decomposed by heat into silver oxide and carbonic acid 
gas. Silver carbonate in solution has an alkaline reaction, 
turning red litmus blue. 


De Pitteurs, Chem. Centr. 1884, p. 411. 


142 


THE CHEMISTRY OF PHOTOGRAPHY. 


Silver Chloride. 

Formula, AgCl : Combining weight, 143.5. 

Silver chloride occurs in waxy, translucent masses called 
‘‘ horn-silver ” in the mines of Mexico, Peru, Chili, and the 
Harz Mountains. It is also obtained as a curdy- white precipi- 
tate when hydrochloric acid, or any soluble chloride, is added 
to a solution of a silver salt. 

For example, the paper used in printing the ordinary posi- 
tive pictures in photography is coated with silver chloride, 
which is produced by floating the paper (previously impreg- 
nated with ammonium or sodium chloride) upon a solution of 
silver nitrate : 

NaCl + AgNOs = AgCl + NaNOj 

Sodium / combines J Silver to Silver I and j Sodium 

Chloride j with { Nitrate form Chloride \ | Nitrate. 

Pure silver chloride is white, but under the influence of light 
it darkens, passing through various tints of violet until it be- 
comes black. 

Silver chloride is insoluble in water and dilute acids. It is 
dissolved by sodium hyposulphite, ammonia, potassium cyanide, 
and by strong solutions of alkaline chlorides and mercuric 
nitrate. Silver sub-chloride (AggCl) has recently been pre- 
pared by M. Guntz. He first obtains silver sub-fluoride 
(AggF) by heating powdered silver with a solution of silver 
fluoride ; and then changes this into the sub-chloride by the 
addition of hydrochloric acid. 

Silver Chromate. 

Formula, AggCrO^: Combining weight, 332J. 

This compound may be obtained by adding a solution of 
potassium bichromate or chromate to a solution of silver 
nitrate. A reddish-brown precipitate is produced, which is 
silver chromate ; this may be filtered off, washed, and dried. 
It dissolves in hot dilute nitric acid, and separates out on cool- 
ing in small ruby-red crystalline plates. Paul Poy used silver 
chromate in the preparation of an emulsion in 1881 (see British 


CHEMICALS EMPLOYED IN PHOTOORAPHY. 


143 


Journal Photo. Almanac^ 1882), and W. K. Burton (Almanac.^ 
1888), points out that it might be used (from its deep ruby 
color) to prevent halation, and also as the actual sensitive salt 
in an emulsion. 

Silver Citrate. 

Formula, AggCgllgO.^ : Combining weight, 513. 

May be obtained as a white precipitate by adding silver 
nitrate to sodium citrate. 

It is insoluble in water, but boiling water decomjDoses it, 
with separation of silver. 

Silver Fluoride. 

Formula, AgF : Combining weight, 121. 

Prepared by dissolving silver oxide or carbonate in hydro- 
fluoric acid, and evaporating the solution. It is readily soluble 
in water (in which it differs from the haloid salts of silver) and 
even deliquesces by absorption of water from the atmosphere. 

Silver Hyposulphite. 

Formula, Ag 2 S 2 03 : Combining weight, 328. 

This compound — more correctly called silver thiosulphate — 
is a snow-white powder, obtained by adding dilute silver 
nitrate to a strong solution of sodium hyposulphite. The pre- 
cipitate is contaminated with silver sulphide, from which it 
may be separated by dissolving in ammonia. On carefully 
neutralizing the ammoniacal solution with nitric acid, the sil- 
ver hyposulphite is again thrown down. 

It has a sweet taste, is but slightly soluble in water, and is 
— in the moist state — very unstable, decomposing into silver 
sulphide and sulphuric acid. 

With hyjiosulphite of soda the silver hyposulphite combines 
to form two double salts. The first of these — AgHaS^Og — is 
produced when the silver salt is in excess ; it is nearly insolu- 
ble in water. The second — AgoHa 4 (So 03 ) 3 — is formed when 
there is an excess of the soda ; it is very soluble in water. In 


144 


THE CHEMISTRY OF PHOTOGRAPHY. 


all fixing operations it must clearly be our aim to produce the 
second (or soluble) salt. 

Silver Iodate. 

Formula, AglOg : Combining weight, 283. 

Prepared by adding potassium iodate to silver nitrate. 

Silver Iodide. 

Formula, Agl : Combining weight, 235. 

Silver iodide is very rare as a mineral, but it is readily pre- 
pared by adding potassium iodide to a solution of a silver salt. 
In the daguerreotype process, it is prepared by the direct 
combination of its elements, a plate of silver being exposed to 
the vapor of iodine. 

Ag + I = Agl 

Silver combines ivith Iodine to form Silver Iodide. 

Unlike silver bromide and chloride, the iodide is insoluble 
in ammonia, which, however, turns it white ; its normal color 
being yellow. When heated, the yellow color deepens. Pure 
silver iodide is not affected by light, but in the presence of a 
little silver nitrate, or any other iodine absorbent, the silver 
iodide darkens, becoming first brown and then grayish-black. 

Silver JSTitrate. 

Formula, AgNOg : Combining weight, 170. 

Silver nitrate can be made by dissolving silver in an equal 
weight of nitric acid, adding water and evaporating the solu- 
tion, when the salt appears as colorless crystals, having a spe- 
cific gravity of 4i. They are soluble in their own weight of 
cold water, the solution being neutral. Silver nitrate is known 
in surgery as lunar caustic^ and is used to destroy proud flesh, 
etc. It is very poisonous. Pure silver nitrate is not affected 
by light unless organic matter be present, when it speedily 
darkens. 

Silver nitrate is very largely used in photography, and it is 
fortunate that it can be purchased at a price but little exceed- 


CHEMICALS EMPLOYED IN PHOTOGKAPHY. 


115 


iiig the value of the silver which it contains. The reason of 
this is that the salt is produced, as a hye-product, in the sepa- 
ration of gold from silver hy the refiners. But very cheap 
silver nitrate is almost certain to contain impurities — such as 
copper, and organic matter — whose presence would spoil the 
salt for photographic purposes. To remedy this the suspected 
crystals should be dissolved in distilled water, and the liquid 
evaporated. The re-crystallized salt will be pure. 

Enormous quantities of silver nitrate are used in the manu- 
facture of our modern gelatine dry-plates. The great English 
makers of these dry-plates usually buy the silver nitrate in 
quantities of ten thousand ounces at a time. 

To find the exact amount (without calculation) of silver 
nitrate required to combine with the soluble bromide which is 
added to it to make an emulsion, Mr. W. Ackland has invented 
a very useful form of slide-rule. 

Silver Nitrite. 

Formula, AgN 03 : Combining weight, 154. 

Silver nitrite is best obtained by mixing equal j)arts of 
strong warm solutions of silver nitrate and potassium nitrite. 
The salt produced is a white crystalline powder, difiEicultly 
soluble in cold water, soluble in hot water with partial decom- 
positiou. By a moderate heat it is decomposed into silver, 
silver nitrate and nitric oxide. AgNO^ has been added to 
the silver nitrate bath used in the wet-collodion process with 
advantage as regards increased sensitiveness and density of 
the wet-plate, but with disadvantage as regards the production 
of fog. 

Silver Oxide. 

Formula, Ag^O: Combining weight, 232. 

Silver oxide may be prepared by adding potassium hydrate 
to silver nitrate. It is a brownish-black powder, one part of 
which dissolves in three thousand parts of water, the solution 
being alkaline. Silver oxide should be kept in water in an 
opaque bottle. Treatment with strong ammonia converts it 
mio fulminating silver^ a highly explosive substance. 


146 


THE CHEMISTEY OF PHOTOGRAPHY . 


Silver oxide is used in the collodion process to neutralize a 
too acid bath of silver nitrate. It has also been employed to 
separate copper oxide from silver nitrate. 

Silver Phosphate. 

Formula, Ag3P04: Combining weight, 419 . 

This substance is thrown down as a yellow powder when 
silver nitrate is added to any normal alkaline phosphate. It 
is insoluble in water, but dissolves in nitric acid and in ammo- 
nia. It blackens when exposed to light, and becomes red 
when heated. 

Silver Sodium-Hyposulphite. 

Formula, AgHaS2 03+2H2 0 . Combining weight, 

243 + 36 = 279 . 

This salt — more properly called silver sodium-thiosulphate — 
can be prepared by adding an excess of a neutral solution of 
silver nitrate to a solution of hyjDOSulphite of soda, when it 
appears as a brown precipitate. It is but slightly soluble in 
water. 

If, on the contrary, an excess of a solution of hyposulphite 
of soda be added to a solution of silver nitrate or chloride, no 
precipitate will be produced, for a compound of silver and 
sodium will then be formed which is very soluble in water. 
Its formula is Ag 2^^4(830 3) 3. This is the salt which is, or 
ought to be, formed in all fixing operations, whether of nega- 
tives or prints. Any given quantity of hyposulphite of soda 
is able to dissolve about one-third of its weight of silver chlo- 
ride. If less of the hypo be employed, the insoluble double 
salt will be formed, and will appear as small crystals on the 
surface of the paper or glass. 

Silver Sulphate. 

Formula, Ag2S04: Combining weight, 312 . 

Prepared by dissolving silver in hot strong sulphuric acid, 
or by dissolving silver nitrate or carbonate in dilute sulphuric 
acid. Silver sulphate forms small lustrous crystals which dis- 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


147 


solve in two hundred parts of cold, or sixty-eight parts of hot 
water. The addition of a little sulphuric or nitric acid to the 
water much increases the solubility. 

Silver Sulphide. 

Formula, AggS : Combining weight, 248. 

This compound, formerly known as sulphuret of silver, 
occurs as a mineral called silver glance. It can be made by 
fusing together silver and sulphur, and is precipitated as a 
black powder when sulphuretted hydrogen is passed into solu- 
tions of silver salts. It is insoluble in water and ammonia, 
but soluble, with decomposition, in nitric acid, by which it is 
converted into sulphate and nitrate of silver. 

Sodium. 

Symbol, Ha: Combining weight, 23. 

Metallic sodium was first obtained by Davy, in 1807, by 
decomposing caustic soda by a strong current of electricity. 
Sodium is never found naturally in the free state, but in com- 
bination with other substances it is one of the most widely 
diffused of the elements. It is a silvery metal, which has so great 
an affinity for oxygen that it tarnishes immediately on exposure 
to the air. Similarly it decomposes water to obtain oxygen. 
A rough but sure test for sodium and its compounds is the 
golden yellow color they produce when placed in the colorless 
flame of the Bunsen burner or spirit-lamp. Becent improve- 
ments by Mr. H. Y. Castner have reduced the price of sodium 
from five shillings to about one shilling per pound. He pre- 
pares a carbide of iron by coking iron with pitch, and mixes 
this with fused caustic soda. When this mixture is heated 
the metallic sodium distills over. The process is being worked 
on a large scale at Oldbury, near Birmingham, Eng., and the 
sodium is used in the manufacture of aluminium. 

Sodium Acetate, 

Formula, HaCgHgOg + 3 II 2 O : Combining weight, 

82 + 53=136. 

Prepared by the action of dilute acetic acid on sodium car- 


148 


THE CHEMISTRY OF PHOTOGRAPHY. 


bonate. Commercially it is made by adding soda to pyrolig- 
neous acid. It dissolves in three parts of cold or one of boiling 
water ; in absolute alcohol it is almost insoluble. Sodium 
acetate forms large, colorless prismatic crystals, which do not 
deliquesce like those of potassium acetate. It is much used 
in the preparation of the gold acetate bath for toning prints. 

Sodium Bicarbonate. 

Formula, Hl^aCOg : Combining weight, 84. 

Natural deposits of this salt are found in Africa, where it 
is called trona^ and in South America, where it is known a& 
ttrao. It can be prepared by passing carbonic acid gas into a 
saturated cold solution of the normal carbonate. Nag CO 3 . 
The bicarbonate of soda is a crystalline white powder, soluble 
in about ten parts of water, and with a feebly alkaline taste 
and reaction. 

Sodium Bi-Borate (Borax). 

Formula, NagB^O^ + IOH 3 O : Combining weighty 

202 + 180=283. 

Borax has been in use from very ancient times as a flux. 
In chemical analysis it is used to detect certain metals by the 
characteristic colors which their oxides impart to ‘‘ borax 
beads.” Borax is now made by boiling crude boric acid 
(obtained from certain lagoons in Tuscany) with sodium car- 
bonate. It is soluble in twenty parts of cold or six of boiling 
water, and the solution has an alkaline reaction. 

In photography, borax is used in the preparation of a toning- 
bath for prints. 

Sodium Bromide. 

Formula, NaBr: Combining weight, 103. 

Prepared by neutralizing hydrobromic acid with sodium 
carbonate. From hot solutions it crystallizes in anhydrous 
cubes; from solutions below 90 deg. Fahr. in prismatic crys- 
tals, containing two molecules of water — NaBr + 2 HgO. It is 
freely soluble in water and in alcohol. 


CHEMICALS EMPLOYED IN PHOTOGKAPHY. 


149 


Sodium Carbonate. 


Formula, NagCOg + lOH^O : 


Combining weight, 


106+180-286. 


Immense quantities of soda ash are produced annually in 
South Lancashire — the alkali district— by treating salt with 
sulphuric acid, and then heating the product (sodium sulphate) 
with powdered coal-slack and chalk. 

From the hlach ash so produced, the impure sodium car- 
bonate {soda ash) is dissolved out with w^ater. 

It is then purified by dissolving again in water, and re-crys- 
tallizing, when large transparent crystals — called soda crystals 
— of sodium carbonate are obtained. These are largely 
used to soften the water employed for washing clothes, etc. 
The crystals dissolve in two parts of cold, or in less than their 
own weight of boiling water. The solution has a strongly 
alkaline taste and reaction. In the United States, washing- 
soda crystals are known as ‘Ual soda.” 

As the alkaline ingredient of the pyro developer, carbonate 
of soda is, by many photographers, preferred to ammonia. It 
should be purchased at the chemist’s as “ pure carbonate of soda 
in crystals.” It is also sold as a dry white powder — ‘Exsiccated 
carbonate of soda” — which is much stronger, because the water 
of crystallization (lOlIgO) has been driven off by heat. 


Common salt occurs plentifully in nature in sea M"ater, in 
salt springs, and as rock salt. When the chemically pure 
sodium chloride is required, it is made by passing hydrochloric 
acid gas into a solution of common salt ; or by neutralizing the 
same acid with carbonate of soda. Sulphate of sodium and 
magnesium chloride are the most common impurities. Sodium 
chloride crystallizes in cubes. It is almost equally soluble in 
hot and in cold water, but is insoluble in alcohol. 

* A little common salt — about one ounce to each pound of 
chlorate of potash — is useful in making oxygen for lantern work. 
It appears to cause the gas to be given off more regularly. 


Sodium Chloride (Common Salt). 


Formula, NaCl : 


Combining weight, 58^. 


150 


THE CHEMISTRY OF PHOTOGRAPHY. 


Sodium Hydrate (Caustic Soda). 

Formula, HaHO : Combining weight, 40. 

This salt is formed when sodium is dissolved in water, but 
most of that used in commerce is obtained as a bye-product 
in the manufacture of sodium carbonate. 

Sodium hydrate is a white, fibrous solid. It melts below a red 
heat, without decomposition, and is usually cast into sticks for 
sale. It is a powerful alkali, and is largely used in soap making. 

Sodium Hypochlorite. 

Formula, HaOCl : Combining weight, 74|-. 

This substance is difficult or impossible to obtain in the 
pure state, but it is contained in the bleaching liquid formed 
by passing chlorine into caustic soda. This liquid was for- 
merly known as Eau de Labarraque. An easier method of 
preparing it is to dissolve four ounces of sodium carbonate in 
ten ounces of hot water ; and two ounces of hypochlorite of 
lime (commonly called chloride of lime,” or bleaching 
powder) in thirty ounces of water; and then mix the two 
solutions, boil, and filter. 

When carbonate of potash is used, practically the same 
result is obtained, but the liquid is then known as ‘‘ Javelle 
water,” or Eau de Javelle. Exactly the same quantities of 
each substance may be used. These solutions should be kept 
in stoppered bottles. They are useful for removing all traces 
of sodium hyposulphite from negatives or prints. For this 
purpose about half an ounce of either solution should be 
mixed with twenty ounces of water. 

Sodium Hyposulphite (‘^Hypo”). 

Formula, AAgSgOg+SHgO : Combining weight, 

15890+=248. 

Some acids contain sulphur in place of oxygen. In recent 
times these acids have beeii distinguished by the prefix thio 
(Greek for sulphur), so that wx now speak of thiosulphuriG 
instead of hyposulphurous acid. As a consequence of this 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


151 


change, sodium liyposulphite — which is a salt of thiosulphuric 
acid — is now properly called sodmm thiosulphate. But the 
old name — and its familiar abbreviation hyjio ” — still com- 
mand most adherents among photographers generally. 

Sodium hyposulphite is prepared on a large scale, and very 
cheaply, from soda waste^ the insoluble matter which remains 
after the extraction of sodium carbonate from hlach ash. It 
is readily soluble in water, and rather deliquescent. Paper 
manufacturers use a great deal of hypo ” as an anti-chlor,” 
to remove the excess of chlorine which they use to bleach the 
vegetable fibres they employ ; the consequence is that ordinary 
paper, white blotting-paper, and card-board contain a little 
sodium hyposulphite, and photographs mounted on such sup- 
ports will be pretty sure to fade. Sodium hyposulphite in 
solution is best kept in a blue bottle and in a tolerably dark 
place. It is a good plan to keep a lump of chalk in the solu- 
tion, as it neutralizes any trace of acid which may be formed. 
When kept in a white bottle, and exposed to sunlight, the 
hypo is slowly oxidized. A mixture of alum aud hypo solu- 
tions rapidly decomposes, the sulphur being separated, and 
causing the mixture to become milky. For this reason gela- 
tine plates that have been soaked in alum must be well washed 
before placing them in the hypo,” or the sulphur will be 
deposited in the film. 

Vessels used in the photographer’s laboratory to hold hypo- 
sulphite of soda should never be employed for any other pur- 
pose. They become so saturated with the fiuid — which will 
pass right through a porcelaiu dish in a few days— as to con- 
taminate every other fiuid put into them. 

Acids decompose “ hypo,” liberating free sulphur, which is 
deposited upon them, and is very injurious to photographic 
■negatives or prints. For this reason the hypo solution must 
always be kept neutral or slightly alkaline. 

Sodium Iodide. 

Formula, Flal : Combining weight, 150 . 

Prepared by neutralizing hydriodic acid with sodium car- 


152 


THE CHEMISTRY OF PHOTOGRAPHY. 


boiiate. From hot concentrated solutions of this salt the crys- 
tals formed are anhydrous cubes ; but if the solutions are 
evaporated at the ordinary temperature, prismatic crystals are 
formed which contain two molecules of water of crystalliza- 
tion~N aI+2 H 3 O . 

Sodium FTitrate. 

Formula, NaNOg : Combining weight, 85. 

Immense natural deposits of sodium nitrate occur in Chili 
and Peru, and hence this salt is commercially known as Chili 
saltpetre. It is purified by dissolving and re-crystallizing the 
natural product ; but after this operation it still contains a 
little sodium chloride and sulphate. These may be got rid of 
by precipitating the nitrate from a boiling saturated solution 
by means of nitric acid. Sodium nitrate is deliquescent, 
absorbing moisture from the air. Hence it cannot be used to 
replace the more expensive potassium nitrate in the manufac- 
ture of gunpowder. Patents have been taken orit for pre- 
venting this deliquescence by covering each particle of the 
sodium nitrate with a coating of paraffin ; but in practice this 
method was not successful. 

Sodium Silicate. 

Formula, Ha3Si4 0g : Combining weight, 302. 

Since the molecule of this substance contains four atoms of 
silicon it may be called sodium tetrasilicate^ but it is usually 
known as silicate of soda, or soluble glass. It can be obtained 
by dissolving powdered flint under pressure in hot, strong 
caustic soda, or by heating sand with soda-ash and charcoal. 
When powdered up it is readily soluble in boiling water, and 
forms a tliick, viscid liquid. It is used in fresco painting, as 
a cement in the manufacture of artificial stone, and in soap 
making. 

Sodium Sulpii-Antimoniate (Schlippe’s Salt). 

Formula, NagSbS^ : Combining weight, 318. 

When 18 parts finely-powdered antimonious sulphide, 17 
parts dry sodium carbonate, 13 parts slaked lime, and 3-^ parts 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


153 


sulplmr are boiled together for some hours in a quantity of 
water, sodium sulph-antimoniate is formed. The liquid must 
then be filtered, and the filtrate evaporated, when Schlippe’s 
salt will be obtained in beautiful crystals. 

Schlippe’s salt can be used to intensify negatives. It may 
either be applied after a solution of bichloride of mercury, or 
(better) after the plate has been soaked in iodide of mercury. 

• 

Sodium Sulphite. 

Formula, NagSOg+THgC^ • Combining weight, 

126 + 126=252. 

Prepared by taking a saturated solution of sodium carbon- 
ate, dividing it into two parts, saturating one part with sul- 
phurous acid, and then adding the other part. On evapo- 
rating, transparent crystals of sodium sulphite are formed. 
These crystals are very soluble in water, slightly soluble in 
alcohol. 

Sodium sulphite combines eagerly with oxygen, becoming 
converted into sodium sulphate, i^agSO^. On this fact 
depends its use in the developer, since it takes possession of 
the oxygen which would otherwise go to the pyrogallic acid. 
It was first introduced for this purpose by Mr. Berkeley, and 
it is recommended to add four times as much sodium sulphite 
by weight as there is pyro employed. By this means a solu- 
tion of pyrogallic acid can be made up and j^reserved for use 
for months, if not years, while without the klagSOg the pyro 
would rapidly discolor and become useless. 

Sodium Tungstate. 

F ormula, ]^a 3 WO 4 +2H ^ O : Combining weight, 

292+36=330. 

Prepared commercially by fusing wolfram (a tungstate of 
iron and manganese which is a fairly common ore) with soda- 
ash. It crystallizes in narrow prisms, which dissolve in four 
parts of cold or two of boiling water. The solution is alka- 
line, and has a bitter taste. The crystals are insoluble in 
alcohol. Sodium tungstate is used to render fabrics uninflam- 


1 


154 THE CHEMISTRY OF PHOTOGRAPHY. 

mable. In photography it has been employed to render the 
gold toning-bath alkaline. 

Starch. 

Formula, : Combining weight, 162. 

Starch forms a large part of every plant. It occurs in the 
form of minute granules^ which are insoluble in cold water, 
alcohol, and ether. In hot water the granules swell and burst, 
forming starch paste ; and by continued boiling the starch 
may be dissolved. Iodine forms a blue compound with starch, 
the color disappearing with heat, but returning when the 
solution is cooled. 

Strontium Chloride. 

F ormula, Sr Cl g + 6 H 3 O : Combining weight, 

158^+1 08=266i. 

Chloride of strontium is formed when strontium carbonate 
(the mineral called strontianite) is dissolved in hydrochloric 
acid. It can be obtained (by crystallization) in long hexag- 
onal needles which deliquesce in air, are very soluble in water 
and in alcohol. By heat the water of crystallization is driven 
off, and the salt remains as a white powder. 

Calcium chloride is frequently present, as an impurity, but 
this may be removed by repeated dissolving in, and re-crys- 
tallization from hot water. 

Sugar (Sucrose). 

Formula, C ^ 3 II 3 3 O ^ ^ : Combining weight, 342. 

Cane-sugar is obtained by boiling the sweet juice of the 
sugar-cane until the sucrose crystallizes out. The transparent 
colorless crystals of sugar are soluble in one-third of their 
weight of water, less soluble in alcohol. When heated with 
silver and mercury salts, cane-sugar reduces them, and it pre- 
cipitates gold from the chloride. Sugar-water dissolves lime 
much more rapidly than pure water. Glucose^ or grape-sugar, 
is represented by the formula Cgllj It is less sweet and 

less soluble than cane-sugar. It is largely present in most 
ripe fruits. 


CHEMICALS EMPLOYED IN' PHOTOGRAPHY. 


155 


Sulphuric Acid, 

Formula, II g SO 4 : Combining weight, 98. 

Sulphuric acid may be considered as sulphur trioxide (SO 3 
— a white crystalline solid) combined with one molecule of 
water. It is prepared by burning sulphur, or some ore of 
sulphur in air, and passing the gas thus formed (sulphur 
dioxide, SO 3 ) into a leaden chamber, where it meets wdth 
nitric peroxide and steam, and is converted into sulphuric acid. 
The common or commercial acid thus obtained has a slightly 
gray tint, and usually contains small quantities of arsenic 
(from the ore) and lead. The pure acid is obtained from the 
commercial by distillation in platinum or glass retorts. 

Commercial sulphuric acid is commonly called oil of vitriol. 
It is a heavy, oily liquid (specific gravity 1.84), boiling at 
640 deg. Fahr. It burns the skin, clothes, or indeed almost 
any organic substance upon which it falls, blackening them 
at the same time. This is due to the fact that it extracts 
water from these substances, leaving their carbon behind. 
When mixed with water the heat produced may exceed 
boiling-point, and for this reason the mixing should always 
be done in thin glass vessels or jugs, the acid being always 
added to the water^ in small quantities at a time, and with 
frequent stirring. 

The white precipitate which appears is sulphate of lead, 
which is insoluble in the dilute acid ; it may be allow^ed to sink 
to the bottom, and the clear liquid then poured ofi. Owing to 
its power of absorbing water, sulphuric acid is often used for 
drying substances without heat ; the substance and the acid 
being placed in separate dishes under a glass shade. 

Sulphuric acid dissolves all the ordinary metals except gold 
and platinum. Hence it is often used for tlie separation of 
gold from silver, the latter being dissolved, and the former 
left behind as a dark powder. 

The presence of sulphuric acid, or any soluble sulphate, in a 
solution, may be detected by adding a few drops of barium 
chloride, when a white precipitate, insoluble in nitric acid, will 
be formed. 


156 


THE CHEMISTEY OF PHOTOGEAPHY. 


SuLPHHEous Acid. 

Formula, IlgSOg : Combining weiglit, 82. 

The true sulphurous acid is formed by dissolving sulphurous 
acid gas (SOg) in water. This gas — also known as sulphurous- 
anhydride, and as sulphur-dioxide — is best obtained by the 
action of sulphuric acid on copper, aided by a gentle heat. 
One volume of water, at ordinary temperatures, dissolves fifty 
volumes of the gas. The solution turns blue-litmus red, and 
has a sour taste. 

Sulphurous acid forms a series of salts called suljpJiites^ 
which are easily decomposed by the stronger acids, SO 3 being 
liberated. 

Tanxic Acid (Tannin). 

Formula, 411 ^ qOq : Combining weight, 322. 

The nut-galls of the oak, excrescences produced by the action 
of insects, contain nearly half their weight of tannic acid, 
which is extracted by soaking the powdered galls in washed 
ether. From its origin it is sometimes called gallo-tannic acid. 
When the ether is subsequently evaporated the tannin is 
obtained as a yellowish amorphous substance, very soluble in 
water, less soluble in alcohol. It has a strongly astringent 
taste, and reddens blue-litmus. 

W ith the ferric, or per-salts of iron, tannic acid gives a black 
precipitate, which is common writing-ink. When exposed to 
the air, or when treated with dilute acids, tannic acid is decom- 
posed into gallic acid, and glucose. It yields pyrogallic acid 
when heated to a temperature of 400 deg. Fahr. 

Gelatine and albumen are precipitated by tannin ; with the 
former it produces a tough material, which is practically 
leather. 

Taetakic Acid. 

Formula, C 4 lIgOg : Combining weight, 150. 

When grape-juice is fermented — as in the manufacture of 
wine — it deposits an impure acid potassium tartrate, which is 
known as lees, tartar or argol, and from which tartaric acid is 


CHEMICALS EMPLOYED IN PHOTOGKAPHY. 


157 


made bj the addition first of chalk, then of calcinm chloride, 
and lastly of sulphuric acid. 

Tartaric acid forms large prismatic, colorless crystals, soluble 
in half their weight of water, and in alcohol. The aqueous 
solution does not keep well. 

Thiosulphuric Acid. 

Formula, Il^SgOg : Combining weight, 114. 

This acid, formerly called hyposulphurous acid, is not known 
in the free state, but .salts of it exist ; the sodium salt 
NagSgOg+bllgO is the familiar ‘‘hypo” of the photog- 
rapher. 

IlRxiNiuM Nitrate. 

F ormula, U O g (NO 3 ) g +6 II g O : Combining weight, 

396+108=504. 

Uranium is a rare metal, whose chief source is the ore called 
pitclMende. By treating this ore with nitric acid uranium 
nitrate is obtained. On evaporation it forms fine yellow crys- 
tals which are soluble in half their weight of water, and which 
deliquesce by the absorption of water from the atmosphere. 

y ANADIUM. 

Symbol Y : Combining weight, 51. 

Although known in its ores since 1801, vanadium was first 
isolated by Boscoe in 1867. 

Varnishes. 

A varnish is any liquid matter which, when applied to the 
surface of a solid body, becomes dry, and forms a hard, glossy 
coating impervious to air and moisture. Varnishes generally 
consist of some resinous substance dissolved in a volatile liquid, 
which on evaporation leaves the resin in the form of a film. 
They are generally divided into two classes — oil varnishes and 
spirit varnishes — according to the substance employed as the 
vehicle or solvent. 

For oil varnishes, either linseed oil, or oil of turpentine is 
employed. The drying of oil varnishes is due to oxidation. 


158 


THE CHEMISTRY OF PHOTOGRAPHY. 


For spirit varnishes the solvent is either alcohol (of not 
higher specific gravity than .815), methylated spirits, or naphtha. 
These dry rapidly by evaporation. 

The following resins are largely used in the manufacture of 
varnishes. They are substances which exude spontaneously, or 
from incisions made in the trunks, etc., of trees. They are 
solid, more or less transparent, infiammable, inodorous bodies, 
insoluble in water, but soluble in alcohol. 

Amber. — A yellowish fossil resin found chiefiy on the 
southern shores of the Baltic Sea. It is the gum of a kind of 
pine tree, and is largely used in the manufacture of ornaments, 
mouth-pieces for pipes, etc. It is soluble in chloroform, and 
then forms the basis of several varnishes. 

A^iimS, or gum anime, is a brownish-yellow transparent 
resin, the product of the locust tree of Central America. 

Rosin^ resin^ or colophony ^ is the solid residue remaining in 
the retort after the distillation of common turpentine. 

or gum copal., is a hard resin which exudes from cer- 
tain trees that grow in the East and West Indies. 

Dammar^ or gum dammar., is mostly obtained from a con- 
iferous tree which grows in the East Indies. 

Elemi^ or g%im elemi^ is a pale yellow, semi-trans]3arent resin, 
brittle superficially, but soft and tough within. It is used to 
give toughness to lacquers and varnishes. 

Lac is a resin combined with much coloring matter, which 
results from the puncture of the bark of certain tropical trees 
by an insect — Coccus lacca. Stick lac is the crude resin as 
broken olf the trees. When melted, strained, and spread out 
in thin sheets it is called shellac. This shellac varies in color 
from orange to garnet ; the palest being the most valuable. 

Bleached lac is made by dissolving lac in a boiling solution 
of caustic potash, and then passing chlorine through the solu- 
tion. The lac is then nearly white, and is used for pale 
varnishes. 

Mastic. — A pale-yellowish resin found in transparent 
rounded beads, which soften when chewed. 

Sandarac. — A resin given by two species of tropical trees 
(thuja and juniperus). 


CHEMICALS EMPLOYED IH PHOTOGRAPHY. 


159 


For photographic purposes spirit varnishes are largely 
employed for covering the delicate surface of the gelatine film 
of the negative. They are best prepared by macerating the 
resin in closed bottles or tins. 

Mr. W. Bedford recommends the following varnish for 
negatives : 


Button lac. .. o | pound 

Sandarac 2 ounces 

Methylated spirit h gallon 


Shake up occasionally during a week, by which time the sol- 
uble portion will be taken up ; but avoid heat, as it is better to 
filter off the sediment. 

The common, or Ijrown hard spirit varnish of the shops 
(when good) is made as follows : 


Gum sandarac 3 pounds 

Pale shellac . . 2 pounds 

Spirits of wine 2 gallons 


Dissolve, and add 

Turpentine varnish 1 quart 

Agitate well, strain quickly through gauze, and after a 
month decant the clear portion from the sediment. 

When diluted with an equal volume of methylated spirit, 
this makes a good varnish for negatives. 

Crystal varnish is very useful for maps, prints, and articles 
of paper generally ; but the paper must first be sized. It is 
made by mixing equal parts of Canada balsam and rectified 
oil of turpentine. 

India rubber^ or flexible varnish^ is made by dissolving in 
the cold one and a half ounces of pure (masticated rubber), 
cut small, in one pint of either chloroform, ether (washed), or 
carbon bisulphide. 

Or India rubber shavings (one ounce) may be dissolved by 
gentle heat in rectified mineral naphtha or benzol (one pint) ; 
but this dries badly. 

The following varnishes for negatives are taken from the 
‘‘ British Journal Almanac ” : 


160 


THE CHEMISTRY OF PHOTOGRAPHY. 


No. 1. 

Sandarac 4 ounces 

Alcohol 28 ounces 

Oil of lavender 3 ounces 

Chloroform 5 drams 

No. 2. 

White, hard varnish of the shops 15 ounces 

Methylated alcohol 25 ounces 

This will be found to be a good and cheap varnish if dura- 
bility is not required, as it is easily rubbed up for retouching 
upon, and easily cleaned off. Very suitable for enlarged 
negatives that are not to be retained. 

No. 3. 

Tough, hard and durable. 

Shellac ounces 

Mastic i ounce 

Oil of turpentine ounce 

Sandarac li ounces 

Venice turpentine ^ ounce 

Camphor 10 grains 

Alcohol 20 fluid ounces 

No. 4. 

Sandarac 90 ounces 

Turpentine 36 ounces 

Oil of lavender 10 ounces 

Alcohol 500 ounces 

No. 5. 

This one may be rubbed down, when cold, with powdered 
resin, and gives a splendid surface for retouching. 

Sandarac 2 ounces 

Seeddac 1 to li ounces 

Castor oil 3 drams 

Oil of lavender 1^ drams 

Alcohol 18 fluid ounces 

No. 6. 

Best orange shellac 1^ ounces 

Methylated spirit 1 pint 

Keep in a warm place until dissolved ; then add a large 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


161 


teaspoonful of whiting or prepared chalk. Set aside to clear, 
and then decant. This is specially recommended for gelatine 
negatives. 

Negative Retouching Varnish. 

Sandarac 1 ounce 

Castor oil 80 grains 

Alcohol Bounces 

First dissolve the sandarac in the alcohol, and then add the oil. 
Ground-Glass or Matt- Varnish. 


Sandarac 90 grains 

Mastic 20 grains 

Ether,. 2 ounces 

Benzole i to ounce 


The proportion of the benzole added determines the natare 
of the matt-surface obtained. This varnish must be applied, 
and allowed to dry, without heating the negative. For the 
other varnishes the glass should be gently heated both before 
and after they are applied. 

W ATER. 

Formula, H 2 O : Combining weight, 18. 

Ice melts at 32 degs. F’ahr., and the liquid water passes into 
water-gas (steam) at 212 degrees. When cooling, water steadily 
contracts until it reaches 39 degs. Fahr. (point of greatest den- 
sity) and then slowly expands until it reaches 32 degs. Fahr., 
when it suddenly expands about one-tenth, so that ten cubic 
feet of water form eleven cubic feet of ice. This is the cause 
of the frequent bursting of water-pipes in frosty weather. 

Pure water is a compound of the two gases, oxygen and 
hydrogen, but ordinary water is far from pure. Indeed, it is 
doubtful if perfectly pure water has ever been obtained. 
Ordinary water contains impurities of two kinds : {p) matter 
supmided in the water, as sand, mud, etc. ; and ih) matter 
dissolved in the water, as salts of lime, etc. From matters in 
suspension the water can be freed by filtration ; while the dis- 
solved substances are left behind when the water is distilled 
and re-distilled. 

Ordinary spring water is more or less hard from the pres- 


162 


THE CHEMISTRY OF PHOTOGRAPHY. 


once of salts of lime — usually the carbonate of lime. Hain- 
water is fairly pure in the country, but in towns, where it falls 
through dirty air and over dirty roofs, it is always much con- 
taminated with soot, etc. When the rain-water runs over or 
through the rocks it dissolves some of the materials of which 
they are composed, and these cause it to be hard. 

All rain-water contains carbonic acid gas dissolved out of 
the air, and it is to the presence of this acid that rain-water 
owes its power to dissolve limestone rocks. When the hard 
water is boiled, the carbonic acid gas is driven off, and the 
carbonate of lime is then deposited on the bottom and sides of 
the vessel. Such a deposit, called fur, may be seen inside 
most kettles. Other common impurities in spring or river- 
w’-ater are sulphates of lime and of magnesia, and as these can- 
not be removed by boiling, they make the water jpermanently 
hard. 

The water of shallow wells and of rivers near large towns 
usually contains some suspended organic matter, derived 
chiefly from sewage, which may be a cause of great danger to 
those who drink it. 

For many purposes in photography ordinary tap water is 
sufiiciently pure, as for washing the plates after development, 
washing prints, etc. ; but for making most solutions and for 
mixing the developing solution distilled water is far better. 

To distill water we require a stilly or vessel in which to boil 
the water ; a worm., in which to cool the steam ; and a receiver 
into which the condensed water may pass. These articles are 
frequently made of tin, or better, of copper. 

Zinc Bromide. 

Formula, ZnBrg : Combining weight, 225. 

Prepared by passing bromine vapor over red-hot zinc. It is 
a white, crystalline salt which greedily absorbs moisture, and 
so deliquesces when exposed to the air. 

Zinc Chloride. 

Formula, ZnClg : Combining weight, 136. 

Prepared by dissolving zinc in hydrochloric acid. It is a 


CHEMICALS EMPLOYED IN PHOTOGRAPHY. 


163 


white and very deliquescent substance, very soluble in water 
and in alcohol. From its great affinity for water it is some- 
times employed for removing the elements of that liquid from 
organic compounds. 

Zinc Iodide. 

Formula, Znig : Combining weight, 329. 

When zinc tilings are heated wdth iodine, they combine to 
form, iodide of zinc, a colorless, deliquescent, unstable sub- 
stance. 

All these salts of zinc have an acid reaction, turning blue- 
litmus red. They cause vomiting, which is fortunate, as they 
are strong poisons. 

Zinc Hypochlorite. 

Formula, ZnClgOg : Combining weight, 168. 

This salt of hypochlorous acid may be prepared by adding 
a solution of zinc sulphate to a solution of calcium hypochlorite, 
and then filtering ofi the insoluble calcium sulphate formed. 
In this state it will be mixed with zinc chloride, but this latter 
substance will not interfere with its use as a hypo eliminator. 
Its use in photography depends upon the fact that a solution 
of hypochlorite of zinc will decompose hyposulphite of soda, 
so that it is used to eliminate the hypo from prints after fixing. 
When a neutral solution of hypochlorite of zinc is added in 
excess to a solution of hyposulphite of soda, a mutual reaction 
takes place between the two, sodium hydrogen sulphate and 
zinc chloride being formed. There is a certain amount of 
danger in its use, as it is an unstable body and gives ofi chlo- 
rine on keeping. If this chlorine comes into contact with 
hyposulphite of soda, free hydrochloric acid is evolved, and 
hydrochloric acid in contact with hyposulphite of soda acts 
upon it with deposition of free sulphur, which will be depos- 
ited in the pores of the paper, and will probably combine 
with the silver. 

ZiRcoNiA (Zirconium Oxide). 

Formula, ZrOg : Combining weight, 122. 

Zirconia is a hard, white powder, resembling silica. Com- 


1G4 


THE CHEMISTRY OF PHOTOGRAPHY. 


pressed into cylinders, it has been recommended for use in the 
‘‘ lime-light ” instead of lime. It is, however, extremely diffi- 
cult to obtain pure zirconia. It is a non-conductor of heat, so 
that before the mixed gases it gives a bright white spot of 
light not more than a quarter of an inch in diameter. 

Table of the Principal Substances which are Known to be Acted- 

Upon by Light. 


Substance. 


First Observer. 


Date. 


Silver. 


Nitrate solution, mixed with chalk, gives in 

sunshine copies of writing 

Nitrate solution on paper [ 

Nitrate photographically used \ 

Nitrate on silk 

Nitrate with white of egg 

N itrate with lead salts 

Chloride 

Chloride in the spectrum 

Chloride photographically used 

Chloride blackened 

Iodide 

Iodide by action of iodine (on metallic silver). . 

Iodide photographically used 

Iodide with gallic acid 

Iodide with ferrous sulphate 

Chloride and iodide by chlorine and iodine (on 

metallic silver) 

Bromide 

Bromide by action of bromine (on metallic silver). 

Sulpho-cyanide 

Nitrite 

Oxide with ammonia 

Sulphate 

Chromate 

Carbonate 

Oxalate 

Benzoate • • 

Citrate 

Kinate . 

Borate 

Pyro-phosphate 

Lactate 

Formiates 

Fulminates 

Sulphide by vapor of sulphur (on metallic silver). 
Phosphide by vapor of phosphorus (on metallic 
sflver) 


J. H. Schulze. . . . 

Hellot 

Wedgwood & Davy 

j F ulhame 

( Rumford 

B. Fischer 

Herschel 

f . B. Beccarius. . . . 

Scheele 

Wedgwood 

Lassaigne 

Dav}^ 

Daguerre 

Herschel 

Talbot 

H unt 

Claudet 

Balard 

Goddard 

Groithus 

Hess 

Mitscherlich 

Bergmann 

Vauquelin 

Buchholz 

Bergmann 

Troramsdorf 

Vauquelin . 

Henry and Plisson. 

Rose 

Stromeyer 

Pelouze and Gay- 

Lussac 

Hunt 

Hunt 

Niepce 

Niepce 


1727 

1737 

1802 

1797 

1798 
1812 
1839 
1757 
1777 
1802 
1830 
1814 

1839 

1840 

1841 
1844 

1840 

1826 

1840 

1818 

1828 

1827 

1779 

1798 

1800 

1779 

1793 

1798 

1829 

1830 
1830 

1833 

1844 

1844 

1820 

1820 > 


CHEMICALS EMPLOYED IX PHOTOGRAPHY. 


165 


Table of the Principal Substances, etc . — Continued 


Substance. 


Gold. 


First Observer. 


Oxide 

Chloride on paper 

Chloride on silk 

Chloride in etherial solution 

Chloride with ferro-cyanide and ferri-cyanide of 

potassium 

Chloride and oxalic acid 

Chromate 

Plate of gold and iodine vapor 

Platinum. 

Chloride in ether 

Chloride with lime 

Iodide 

Bromide 

Cyanide., 

Double chloride of platinum and potassium. . . . 


Scheele. . . . 

Hellot 

Fulhame. . 
Rumford. . . 

Hunt 

Ddbereiner 

Hunt 

Goddard. . . 


Gehlen . . . . 
Herschel. . . 
Herschel . . 

Hunt 

Hunt 

Ddbereiner 


Mercury. 

Oxide (mercurous) 

Oxide 

Oxide (mercuric) 

Oxide (more accurate observations). 

Chloride (mercurous) 

Chloride (mercuric) 

Chloric with oxalic acid 

Sulphate 

Oxalate (mercuric) 

Oxalate (mercurous) 

Sulphate and ammonia (mercurous), 

Acetate (mercurous) 

Bromide (mercuric). 


Gay - Lussac and 

Thenard 

Davy. . . 

Davy 

Abildgaard,Harrup 

not till 

R. Neumann, pre- 
vious to 

Boullay . 

Bergmann 

Meyer 

Bergmann 

Harff 

Fourcroy ......... 

Garot 

Ldwig 


Iodide (mercurous) 

Iodide (mercuric) 

Citrate (mercuric) ... . 

Tartrate and Potassium (mercurous), 

Carbonate (mercuric) 

Nitrate 

Sulphite (mercuric) 

Iron. 


j Torosewicz 

I Artus 

Field 

Harff 

Carbonell & Bravo. 

Davy 

Herschel 

Vitruvius 


Sulphate (ferrous) 

Chloride (ferric) and alcohol.. 

Chloride and ether 

Oxalate (ferric) 

Ferro-cyanide of potassium. . . 

Sulpho-cyanide 

Prussian blue 

Ferric Citrate with ammonium 

Ferric Tartrate 

Chromate 


Chastaing 

Bestuscheff 

Klaproth . . 

Ddbereiner 

Heinrich . 

Grotthus 

Scopoli 

Herschel 

Herschel 

Hunt 


Date. 


1777 

1737 

1794 

1793 

1844 

1831 

1844 

1842 


1804 

1840 

1840 

1844 

1844 

1828 


1811 

1812 

1797 

1797 

1801 

1739 

1803 

1776 

1764 

1776 

1836 

1791 

1826 

1828 

1836 

1836 

1836 

1836 

1831 

1812 

1840 

l.B.C. 


1877 

1725 

1782 
1831 
1808 
1818 

1783 
1840 
1840 
1844 


166 


THE CHEMISTRY OF PHOTOGRAPHY, 


Table of the Principal Substances, etc. — Continued . 


Substance. 


First Observer. 


Copper. 

Chloride (cupric dissolved in ether) 

Oxalate with sodium 

Chromate 

Chromate with ammonium 

Carbonate 

Iodide 

Sulphate 

Chloride (cuprous) 

Copper plates (iodized) 


Manganese. 

Sulphate 

Oxalate 

Potassium permanganate 

Peroxide and cyanide of potassium 
Chloride 


Gehlen. . 
A. Vogel 


Hunt 


A. Vogel, 
j Kratoch 
\ Talbot.. 


Brandenburg 

Suckow 

Frommberg. . 

Hunt 

Hunt 


Lead. 


Oxide 

Iodide and sulphite 

Peroxide 

Red lead and cyanide of potassium 
Acetate 


Nickel. 

Nitrate 

Nitrate with ferro-prussiates. 
Iodide 


Tin. 

Purple of Cassius 

Various Substances. 

Cobalt 

Arsenic sulphide (realgar). 

Antimony sulphide.. 

Bismuth salts 

Cadmium salts 

Rhodium salts 

Vanadic sal^s 

Iridium ammonium-chloride 

Potassium bichromate 

Potassium with iodide of starch. . . 

Metallic chromates 

Chlorine and hydrogen 


Chlorine (tithonized) . 
Chlorine and ether. . . . 

Chlorine in water 

Chlorine and ethylene 


Chlorine and carbon monoxide 

Chlorine and marsh-gas 

Chloride and hydrocyanic acid. 


Davy 

Schdnbein. 

Gay-Lussac 

Hunt 

Hunt 


Hunt 


Uncertain 


H unt. 

Sage 

Suckow 

Hunt 

Roscoe 

Ddbereiner 

Mungo Ponton. . . . 

Becquerel 

Hunt 

Gay-Lussac and 

Thenard 

Draper 

Cahours 

Berthollet 

Gay-Lussac and 

Thenard 

Davy 

Henry 

Serullas 


Date. 


1804 

1813 


1844 


1859 

1841 

1841 


1815 

1832 

1824 

1844 

1844 


1802 

1850 

1811 

1844 

1844 


1844 


1844 

1803 

1832 

1844 

1874 

1831 

1838 

1840 

1843 

1809 
1842 

1810 
1785 

1809 

1812 

1821 

1827 


CHEMICALS EMPLOYED IH PHOTOGEAPHY. 


m 


Table of the Principal Substances, etc. — Continued . 


Substance. 


Various Substances. 

Bromide and hydrogen 

Iodine and ethylene 

Cyanogen, solution of 

Various other methyl compounds 

Hydrocyanic acid 

Hypochlorites (calcium and potassium) 

Uranium chloride and ether 

Molybdenate of potassium and tin salts 

Crystallization of salts under influence of light. 

Phosphorus (in hydrogen, nitrogen, etc) 

Phosphoretted hydrogen 

Nitric acid 

Hog’s fat 

Palm oil 

Asphalt 

Resins (mastic, sandarac, gamboge, ammo- 

niacum, etc) 

Guaiacum 

Bitumens all decomposed, all residues of es- 
sential oils 

Colored extracts from flowers 

Similar coloring matters spread on paper 

Yellow wax, bleached 


Eudoxia macrembolitissa (purple dye) 


Other purple dyes 

Oils generally. . . . 

Nitric ether 

Nicotine 

Santonine 


First Observer. ! 

Date. 

! 

Balard ^ 

1832 

Faraday i 

1821 

Pelouze and Rich- 


ardson 

1837 

Cahours 

1848 

Torosewicz 

1836 

Ddbereiner 

1813 

Gehlen 

1804 


1800 



( Petit 

1722 

■< Chaptal 

1788 

( Dize 

1789 

Bockmann 

1800 

A. Vogel 

1812 

Scheele 

1777 

Vogel 

1806 

Fier 

1832 

Niepce 

1814 

Senebier 

1782 

Hagemann 

1782 

; Daguerre 

1839 

Senebier 

1782 

Herschel 

1842 

1 ( 

1st 

Pliny \ 

cent’y, 

( 

A.D. 

' i 

10th 

1 ( 

cent’y. 

j Cole 

T684 

1 Reaumur 

1711 

^Senebier 

1782 

Senebier 

1782 

'Henry and Bout- 


I ron-Charlard . . . . 

1836 

Merk 

1883 



V 


CHAPTEK XIY. 


CHEMICAL COMPOSITION OF THE SENSITIVE SUR- 
FACES EMPLOYED TO RETAIN THE CAMERA- 
IMAGE IN PHOTOGRAPHY, AxND THE 
CHEMISTRY OF THEIR PREP- 
ARATION. 

The first man who obtained an image upon a sensitive 
surface by the means of a lens, was Hurnpiirey Davy, the 
famous chemist, in or shortly before the year 1 802.* He used 
]>aper coated with silver chloride, and his lenses were those of 
the solar microscope. But silver chloride is much less sensitive 
to light than certain other salts of silver ; and for use in the 
camera it has been displaced first by silver iodide, and then by 
silver bromide. 

lleary Fox Talbot, too, used silver chloride in his “Process 
of Photogenic Drawing” which he published in 1839. He 
made an advance upon Davy’s work, in that he discovered 
that the ])reseiice of silver nitrate (upon the coated paper, and 
intimately associated with the silver chloride) greatly increased 
the sensitiveness to light of the latter substance. He formed 
'the silver chloride in and upon the paper, by soaking the paper 
in a weak solution of common salt, and then brushing it over 
twice with a solution of silver Jiitrate, of strength about sixty 
grains to the ounce. By the mixture and chemical combina- 
tion of these two materials, silver chloride was formed as 
follows : 

NaCl + AgNOo = AgCl + NaNOg 
Sodium and Silver produce Silver and Sodium, 
Chloride Nitrate Chloride Nitrate. 

The slight excess of silver nitrate (due to the second brush- 
ing), acts as a sensitizei\ absorbing and combining with the 

'■ See “ Journal of the Royal Institution,” London ; Vol. I. 


CHEMICAL COMPOSITION, ETC. 


169 


chlorine gas which is given off when the prepared paper is 
exposed to the action of light. 

SAgNOg + CI2 + HgO = 2 AgCl + 2HNO3 + O 
Silver and Chlorine and p^vduce SWvqx ajid Nitric and Oxygen, 
Nitrate Chloride Acid 

But although Talbot succeeded in obtaining images in the 
camera, by means of paper coated in this way, yet the neces- 
sary exposure was very long — from thirty minutes to one hour 
— and its use for this purpose was soon discontinued. It is to be 
noticed that with it Talbot obtained d,])rinted-out negative image 
in the camera; no subsequent process of development being 
necessary, or indeed possible. This sensitive surface of silver 
chloride has formed, however, the principal printing process 
for obtaining positive prints from negatives secured by other 
methods, from Talbot’s time down to the present day. It is, in 
fact, the substance with which ordinary silver ” or sensi. 
tized ” paper is coated. The properties of silver chloride are 
described more fully in the Chapter on The Chemistry of 
Printing.” 

JViepceotype^ or Ileliography . — The first pjermanent pic- 
tures secured by the agency of light, w^ere those obtained by. 
Joseph Nicephore Niepce, of Chalons, about the year 1816 . 
He dissolved bitumen in oil of lavender, and coated metal 
plates with it. The jalates were then exposed in a camera for 
several hours. The effect of light was to render the bitumen 
upon which it acted insoluble in oil of lavender ; so that the 
picture could be developed by subsequent washing with that 
liquid. In repeating this experiment, we find that petroleum 
acts as well as the more expensive lavender oil. 

Bitumen, or asphaltum, is composed of the elements hydro- 
gen and carbon, and is therefore termed a hydrocarbon. By 
exposure to light these elements combine with the oxygen pres- 
ent in the air, or with the moisture in the air touching the plate. 
The oxidized hydrocarbon ” is then insoluble in liquids in 
which the hydrocarbon alone would readily dissolve. The 
change is too complex, and too little of its exact nature is cer- 
tainly known to enable us to represent it by a chemical equa- 
tion. 


170 


, THE CHEMISTRY OF PHOTOGRAPHY. 


Those who wish to repeat ^Niepce’s experiment will find it 
better to expose the plate coated with asphalt beneath a nega- 
tive, rather than in the camera ; the necessary exposure to light 
will then only he ten or fifteen minutes. 

Preparation of the Sensitive Surf ace for the Daguerreotypje 
Process . — In the process published by Daguerre in 1839, he 
obtained a surface extremely sensitive to light by exposing the 
surface of a silver plate (silver plated upon copper was always 
used, to save expense) to the action of the vapor of iodine. 
Iodide of silver was, of course, produced by the combination 
of the two elements : 

Ag + I = Agl 

Silver and Iodine produce Silver-Iodide. 

The iodine was placed at the bottom of a box, and the plate 
of silver was suspended over it. Iodine readily evaporates, 
and in two or three minutes the surface of the silver plate (as 
seen through a little window in the box) had lost its metallic 
lustre, and acquired the line yellow hue of iodide of silver. 
The metallic silver hehind the surface layer of silver iodide 
acted as a sensitizer, absorbing the iodine which was given o£E 
under the influence of light. 

But this surface of pure silver iodide required a rather long 
exposure in the camera — from fifteen to twenty minutes ; and 
it was not until J. F. Goddard discovered, in 1840, that by 
exposing the iodized silver plate to the fumes of the liquid 
non-metallic element bromine the necessary exposure could be 
reduced from minutes to seconds, that the daguerreotype pro- 
cess became a real success. The bromine united with the 
silver iodide to form a compound which may be called bromo- 
iodide of silver, and which was extremely sensitive to light. 
The change was indicated by the yellow surface of the plate 
assuming a rosy hue. 

It is true that Fox Talbot had, in 1839, discovered the great 
sensitiveness to light of silver bromide. But its application 
to the daguerreotype process, and its practical success as a 
diminisher of exposure are due to Goddard. 

How Talhot Prepared his Calotype Paper. — It is probable 


CHEMICAL COMPOSITION, ETC. 171 

that Fox Talbot availed himself of ideas suggested both by 
Daguerre and by an English clergyman named J. B. Beade, 
in the working out of the process which he patented in 1841 
under the name of ‘‘ Calotype ; ” but this in no way detracts 
from the credit due to him for devising so practical and suc- 
cessful a method. He substituted silver iodide for the silver 
chloride which he had previously employed ; and he gave 
only a short exposure in the camera, developing the latent 
image thus impressed by a method which Beade had not 
indeed published, but which he had made known to some of 
his friends, including Andrew Boss, the famous optician. 

By the calotype process, Talbot brushed a solution of silver 
nitrate over paper, which was then dried and dipped into a 
solution of potassium iodide. The strength of the solutions 
was so arranged that there should be an excess of the iodide. 
By the combination of these two chemicals the paper was 
covered with a yellow coating of silver iodide: 

AgNOg + KI = Agl + KNOg 

Silver and Potassium produce Silver and Pota3sium 
Nitrate Iodide Iodide Nitrate. 

In this state the paper was not sensitive to light, for there 
was no sensitizer present to combine with the iodine which 
light would liberate. 

When the iodized paper was required for use, it was brushed 
over with a mixture of gallic acid, acetic acid, and nitrate of 
silver. It might then be exposed while wet, or it could be 
dried and kept for use. The “gallo-nitrate of silver,’’ as the 
mixture just described was called, acted both as a sensitizer 
and as a developer ; the silver nitrate fulfilling the former 
function, while the gallic acid developed the picture. The 
acetic acid played the part of a restrainer. 

The calotype process was much practised — principally by 
amateurs, and for landscape work — from 1841 to 1855, or 
thereabouts. 

Chemistry of the Albumen Process of Niepce de St. Victor. 
— In 1847, the younger Niepce substituted glass plates for the 
paper support used by Talbot in his calotype process. To 
enable the chemicals employed to adhere to the glass, Niepce 


172 


THE CHEMISTRY OF PHOTOGRAPHY. 


used albumen (white of egg), in which he dissolved potas- 
sium iodide, potassium bromide, and sodium chloride (com- 
mon salt). He then converted these three substances into the 
corresponding salts of silver by immersing the coated glass in 
a bath of silver nitrate. 


AgN03 

Silver 

Nitrate 

+ 

and 

KI 

Potassium 

Bromide 

produce 

Agl 

Silver 

Iodide 

+ 

and 

KNO3 

Potassium 

Nitrate. 

AgN03 

Silver 

Nitrate 

+ 

and 

KBr 

Potassium 

Bromide 

produce 

AgBr 

Silver 

Bromide 

+ 

and 

KNO3 
Potassium 
N itrate. 

AgN03 

Silver 

Nitrate 

4 - 

and 

NaCl 

Sodium 

Chloride 

produce 

AgCl 

Silver 

Chloride 

+ 

and 

NaNOg 
Sodium 
N itrate. 


The plates were then ready for exposure (wet or dry), and 
were afterwards developed with a solution of gallic acid. This 
albumen process gave beautiful results, but it was very slow ; 
the usual length of exposure being from ten to twenty minutes. 

Chemistry of the Preparation of Wet Collodion Plates . — 
The wet collodion process was the work of F. S. Archer, in 
1851. It is most interesting to trace the evolution of photo- 
graphic processes; and nothing can be clearer than the fact 
that the collodion process (which reigned supreme from 1851 
to 1879) was the outcome of the calotype and the albumen 
processes. Archer substituted collodion for albumen as a 
means of causing the chemicals to adhere to the glass. He 
lii-st coated the glass plate with collodion in which potassium 
iodide and bromide had been dissolved. The coated plate was 
then dipped into a solution of silver nitrate, when what chem- 
ists call ‘‘double decomposition^’ took place, and silver iodide 
and bromide were formed within and upon the collodion. A 
certain amount of the silver nitrate solution also clung to the 
surface of the plate, and acted as a sensitizer. We may repre- 
sent the action of the silver nitrate as a sensitizer by an equa- 
tion : 


2AgNO, + K + II, O = 2AgI + 2 HNO 3 + O 

Silver a)id Iodine and ^ produce Silver and Nitric and Oxy- 

Nitrate Iodide Acid gen. 


CHEMICAL COMPOSITION, ETC. 


The other equations are the same as those just given for the 
albumen process. The plate was exposed, while still wet, in 
the camera ; and \vas afterwards developed by pouring upon it 
a mixture of pyrogallic acid and acetic acid. 

With more experience in the working of the wet collodion 
process, it was found that the best halogens with which to 
^‘salt” or impregnate the collodion were ammonium iodide 
and cadmium bromide. The chemical changes produced when 
collodion so salted is dipped into a bath of silver nitrate solu- 
tion may be expressed by the following two equations : 


AgNOg 

-f 

NHJ 

= 

Agl 

+ 

NH^NOg 

Silver 

and 

Ammonium 

produce 

Silver 

ajid 

Ammonium 

Nitrate 


Iodide 

Iodide 


Nitrate. 

SAgNOg 

-F 

CdBr^ 

— 

2AgBr 

-V 

Cd(NOg)2 

Silver 

and 

Cadmium 

p7'oduce 

Silver 

and 

Cadmium 

Nitrate 


Bromide 

Bromide 


Nitrate. 


The inconvenience of carrying a portable dark-room, and a 
large glass vessel bath’*) to hold the nitrate of silver solution, 
together with all the other articles necessary to sensitize and 
to develop a wet collodion plate, was very great. For the 
plate had to be exposed while loet. If the silver nitrate solu- 
tion were washed off and the plate dried, it was found to have 
lost much or all of its sensitiveness to light. If the plate were 
dried with the nitrate solution still upon it, the silver nitrate 
crystallized out, forming a network of crystals which s] 3 oilt 
the even surface of the collodion. This drying - ujd of the film 
prevented very long exposures being given in the camera, 
such as were frequently necessary for interiors, etc. More- 
over, it was necessary, after exposure, to develop) the 2 ^ 1 ate 
before it had tune to dry. For these reasons the photographer 
was compelled to di’ag a dark-tent and all the necessary mate- 
rials for sensitizing and developing his plates, about with him. 
W ell may the modern kodakist shudder as he reads of those 
times ! 

Dry Collodion Plates. — Very soon after Archer’s publica- 
tion of the wet collodion process in 1851, attempts were made 
to reduce, or do away with, the necessity for exposing and 
developing the plate while still wet. The first attempts took 


iTtl: THE CHEMISTRY OF PHOTOGRAPHY. 

the form of preventing evaporation, as when M. Grirod applied 
a plate of glass in contact with the wet film, in 1853 ; but it is 
evident that this would be likely to abrade and injure the 
delicate skin of collodion. Then Messrs. Crookes and Spiller, 
in 1854, coated tlie wet collodion with a solution of nitrate of 
zinc. This substance absorbs moisture from the air, and so 
keeps the film from drying up. Then Shadbolt and Lyte, in 
the same year, coated the collodion film with a solution of 
grape sugar, or of honey ; which again kept the surface moist. 
Oxymel — a mixture of vinegar and honey — was recommended 
for the same purpose by J. D. Llewelyn, in 1856. But in 
practice all these methods were found to be but very poor 
makeshifts. In 1857 a Mr. H. N. King managed to keep the 
surface of his plates moist without doubt, for he carried his 
plates in a light-tight box filled with distilled water ! 

Another great trouble to the early experimenters who 
attempted to obtain ‘^dry” collodion plates, was the fact that 
the collodion film when dry had a great tendency to flake or 
scale off from the glass plate. This was obviated in 1859 by 
the introduction by Hardwich, Barnes, and others, of various 
materials, such as gelatine, albumen. India-rubber, etc., with 
which the glass plate was thinly coated before it was covered 
with collodion. Any such adhesive was called a “ substratum,’’ 
and the collodion adhered firmly to it. 

Successful Collodion Dry Plates . — The first succesful dry- 
plate process with collodion was the discovery of Dr. J. M. 
Taupenot, in 1855. He washed the sensitized collodion plate, 
and then flowed it over with iodized albumen ; the plate was 
then again sensitized and again washed ; finally it was dried. 
This process was followed by many others; and many sub- 
stances, such as gelatine, gallic acid, gum arabic, tannin, coffee, 
tea, etc., were used to flow over the previously sensitized and 
washed collodion plates. Such substances received the name of 

preservatives,” because they acted as a kind of varnish, pre- 
serving the surface of tlie film from the injurious action of the 
air ; but they a] so acted as sensitizers, absorbing the halogen 
which was given off under the action of light. Further, 
by filling up the pores of the collodion they offered an easy 


CHEMICAL COMPOSITION, ETC. 


175 


waj of access to tlie film wlien the developer was subsequently 
applied. Perhaps the most successful of the numerous preserv- 
atives was tannin^ which was recommended by the late Major 
Piissell in 1861-65. 

Emulsion Photography. 

An emulsion ” is the name applied to a liquid which con- 
tains innumerable particles of some solid substance, the parti- 
cles being so small and so nearly of the same specific gravity as 
the liquid, that they remain suspended in the hitter for a longer 
or a shorter period of time. Thus milk is an emulsion. It 
consists of countless particles of fat (cream) suspended in a 
watery fiuid. 

As early as 1853 the French worker, Gaudin, sought to re- 
pare a sensitive collodion or jjliotogene^ which could be simply 
poured on to plates or paper, and then dried. The cause of his 
failure — and that of some later exjDerimenters — consisted in the 
use of iodide of silver. When this substance is shaken up in 
collodion, its jiarticles clot together and subside to the bottom. 
This difficulty was overcome in 1861 by B. J. Sayce and W. 
B. Bolton. They formed silver Iromide in the collodion, and 
found that they had got a good “emulsion.” For the par- 
ticles of silver bromide remain suspended in the gelatine very 
much as the fat-globules remain suspended in milk. 

Chemistry of Collodion Emulsion- Making. —Ho make a 
satisfactory collodion emulsion for negative work, it is neces- 
sary to first dissolve a soluble bromide in alcohol and add it to 
some collodion. Silver nitrate is also dissolved in alcohol and 
added gradually to the bromized collodion, which must be 
kept well agitated. Supposing zinc bromide to have been 
employed, the following chemical reaction then takes place : 

2 AgN 03 + ZnBrg = 2AgBr -i- ZnCNOg)., 

Silver and Zinc produce Silver and Zinc 

Nitrate Bromide Bromide Nitrate. 

This equation would show that the silver nitrate and the 
zinc bromide should be mixed together in the proportion of 
340 parts by weight of the former to 225 parts of the latter. 


176 


THE CHEMISTRY OF PHOTOGRAPHY. 


In practice, however, a slight excess of the silver nitrate is 
always employed. 

The zinc nitrate which is formed must be removed from the 
emulsion ; and this is done by washing the emulsion well with 
water, either before coating the plates, or after. It is found 
to be impossible to wash aw^ay all the excess of silver nitrate. 
Some silver nitrate always remains clinging to the molecules 
of silver bromide, and this acts os a sensitizer. Carey Lea 
showed, in 1S70, that it was useful to add a few drops of 
nitric acid to the emulsion, in order to prevent fog. It was 
usual to flow over the coated and washed plate a solution of 
tannin. This did not increase the sensitiveness, but it was 
useful in the other ways we have pointed out. 

Ripening^^ of Collodion Emulsion. — After the ingredients 
of a collodion emulsion had been well shaken up together, it 
was found to be a good plan to leave the emulsion for twenty- 
four hours to “ripen,” as it was called. This ripening consists 
in an aggregation of the molecules of silver bromide, so as to 
form particles of the size most sensitive to light. 

The collodion emulsion dry-plate process was much used for 
landscape work, and by travellers, between 1870 and 1880. 
It was slow — much slower on the average than wet collodion — 
but good work was done with it. 

Chemistry of Gelatine Emulsion-Making. — Several early 
workers attempted to use gelatine, instead of collodion, in the 
preparation of a surface sensitive to light. The first success 
in this direction was due to Dr. D. L. Maddox, in 1871 ; he, 
however, had not time to work out the process so that it should 
be a commercial success. Burgess, in 1873, and Kenneth 
during the years 1876-77, tried hard to introduce gelatine 
emulsion dry-plates to the English market ; but without suc- 
cess. Then came the discoveries of Bennett (1878) and of 
Mansfleld (1879), showing that extraordinary sensitiveness was 
conferred upon a gelatine emulsion when it was carefully 
heated. The increase in rapidity “did the trick” ; and gelatine 
displaced collodion in 1879-80. 

The following short outline of the method by which several 
millions of gelatine dry-plates are now prepared annually will 


CHEMICAL COMPOSITION, ETC. 


ITT 


enable ns to explain the chemical and physical changes which 
take place during this manufacture : 

A. In 8 ounces of distilled water soak 40 grains of gelatine ; 
add 180 grains of ammonium bromide and 10 grains of potas- 
sium iodide. Heat gently till all is dissolved. 

B. In 1 ounce of distilled water dissolve 100 grains of silver 
nitrate; to this add strong ammonia, drop by drop, till the 
precipitate at first formed just disapjDears. 

C. In the dark-room, warm the solution A to ITO deg. F.; 
and add to it, by degrees, 165 grains of silver nitrate. When 
this has dissolved, add solution B. Shake well, and stew for 
two hours at a temperature of ITO deg. F. 

I). Cool the emulsion down to 80 deg. F., and add 300 
grains of hard gelatine. Heat the whole to 100 deg. F., and 
mix well. How place the vessel containing the emulsion in 
cold water, when it will quickly ‘‘ set ” to a stiff jelly. 

B. Wash the emulsion well, by squeezing it through coarse 
canvas into several changes of water. 

B. Add 1 ounce of alcohol to the emulsion ; dissolve it by 
gentle heat; make the total quantity up to 10 ounces by 
adding distilled water. Lastly, filter the emulsion by squeezing 
it through swansdown calico, and it is ready for coating the 
plates. Such an emulsion will possess extremely high sensi- 
tiveness to light. 

The only certain chemical reactions which take place in 
making a gelatine emulsion are those between the silver nitrate 
and the soluble bromide and iodide employed. 

AgNOa + NH4Br = AgBr + NH4NO3 

Silver and Ammonium produce Silver and Ammonium 

Nitrate Bromide Bromide Nitrate. 

AgNOa + KI = Agl + KNO3 

Silver and Potassium produce Silver and Potassium 

Nitrate Iodide Iodide Nitrate. 

Sometimes the potassium iodide is omitted ; but its use is 
generally considered to give additional clearness to the plates. 

The object of washing the emulsion is to get rid of the 
extraneous salts — the nitrates of ammonium and potassium. 

By heating the emulsion, and also by the addition of ain- 


178 


THE CHEMISTRY OF PHOTOGRAPHY. 


moniay we cause the molecules of silver bromide to aggregate 
together until they form particles averaging the eight-thou- 
sandth part of an inch in diameter. It is when they are of 
this size that silver bromide particles are most sensitive to 
light. 

The proof that silver bromide could exist in several distinct 
molecular forms, each differing in sensitiveness to light, was 
first published by the Belgian chemist, Stas, in 1874. Ten 
years later, M. de Pitteurs studied the same subject, specially 
from a photographic point of view ; and the results of his 
investigations are shown in the following table : 

Table showing the Eight Modifications, or Allotropic Forms, of 

Silver Bromide. ' 


By Transmitted Light. 


By Reflected 
Light. 


Semi-trans- 
parent 


■ i 


Almost 

opaque. 


i: 

Slate-blue. 

Orange. \ 

i 

1 

Bluish-white. 

Reddish- \ 

Bluish-white. 

orange. ^ 


1 

Yellowish-white. 

r 

Yellowish-white. 

Violet-blue. -| 
1 


1 

1 

Greenish-yellow. 


Green or violet- 


green. 

Blue. 

Indistinct. 


Occurrence. 


In fresh collodion emul- 
sion. 

Older bromide of sil- 
ver in collodion wet 
plates. 

In very sensitive wet 
collodion plates. 

In very old bromide of 
silver in collodion. 

In very sensitive col- 
lodion emulsion. 

Bromide of silver in 
gelatine : sensitive- 

ness medium. 


Very sensitive gelatine 
emulsion. 


f Slightly sensitive silver 
I bromide in collodion, 

^ yielding indistinct 

I pictures. Affected by 

[ red end of spectrum. 


It is either the sixth or the seventh of these forms of silver 


CHEMICAL COMPOSITION, ETC. 


179 


bromide which the modern plate-maker aims at securing for 
his sensitive drj-plates ; and he is guided to some extent dur- 
ing the preparation of the emulsion by the color of a drop of 
the liquid emulsion when placed on a glass plate. The cause 
of the growth in size of the molecules of silver bromide — and 
of the consequent greater sensitiveness to light — is the fact 
that part of the silver bromide is dissolved by the hot liquid 
in which it is formed, and by the ammonia which is present. 
This dissolved silver is afterwards deposited upon the undis 
solved particles, causing them to increase in size from their 
original diameter (which is about the one twenty-thousandth 
part of an inch) to the one eight-thousandth part of an inch. 
The extremely small particles transmit ruby light ; the larger 
ones transmit blue light. 

It is not at all a difficult task to prepare a good gelatine 
emulsion. To the amateur plate-maker the difficulties lie 
rather in the subsequent work of coating glass plates evenly 
with the said emulsion, and then drying these plates in a per- 
fectly dark room, free from dust, and at a certain rate. 

Gelatine as a Sensitizer. — In our modern gelatine dry-plates 
it is found to be quite unnecessary (harmful, in fact) to have 
any excess of nitrate of silver present to act as a sensitizer ; 
neither is it requisite to coat the plates with tannin, or with 
any other preservative. The fact is that gelatine is itself able 
to act as a “ sensitizer,” and to combine with the small quanti- 
ties -of bromine and of iodine which are given off when sun- 
light acts upon such a plate. This is the cause of the great 
superiority in sensitiveness of gelatine dry-plates over collodion 
or albumen. The gelatine is (comparatively) a powerful sensi- 
tizer. The German chemist, Knop, found * that gelatine 
could combine with nearly one-third its weight of bromine, 
forming a yellowish insoluble bro mo-gelatine. 

Celluloid as a Suj)port for Gelatine Emulsion. — In 1856 a 
Birmingham chemist named Barkes succeeded in converting a 
variety of gun-cotton into a horny substance which was named 

celluloid.” In 1888 certain American manufacturers pre- 


* Chem. Centralblatt^ i8jq. 


180 


THE CHEMISTEY OF PHOTOGRAPHY. 


pared a transparent kind of celluloid, and this is now largely 
used by professional plate-makers in lieu of glass as a support 
for the gelatine emulsion. The Eastman Company use cellu- 
loid which is only the four-hundredth part of an inch in thick- 
ness. This thin celluloid is quite flexible, and after coating, it 
is wound into rolls, which are used with a special piece of 
apparatus called a roll-holder,” which is fitted to the back of 
the camera. Other makers use a stouter kind of celluloid, 
which lies in the dark-slide just like a sheet of glass. The 
advantages of celluloid over glass are, of course, its greater 
lightness and non-liability to breakage. 



CHAPTER XY. 


THE CHEMICAL ACTION OF LIGHT— NATURE OF 
THE LATENT IMAGE. 

The action of light upon the salts of silver is perhaps the 
most difficult and vexed question in the chemistry of photog- 
raphy. Exposure to sunlight for even the ten-thousandth part 
cf a second produces a change in the bromide of silver with 
which our dry-plates are coated. With short exposures like 
this, no visible change is produced ; but by the action of cer- 
tain chemicals, the invisible, latent, or photographic image can 
be developed^ and made visible as a dark-colored substance. 

But a visible imao-e, indistino^uishable from either the latent 
or the developed image by all the tests which we are able to 
apply, is obtained — is ‘‘printed out,” as we say — when the 
plate is exposed for a much longer time (half an hour, or more) 
to the action of light. It has happened to us more than once, 
that, after a very long exposure, a portion of the image (a 
window for examj^le) has been visible as a dark patch when 
the plate has been removed from the dark-slide. 

The question which we have now to consider is, what is the 
chemical composition (1) of the latent image, (2) of the devel- 
oped image, and (3) of the printed-out image % 

It may be assumed — though it has not been absolutely 
proved — that these three images are of the same nature, and 
the same chemical composition. Eight or ten theories have 
been advanced, and we shall proceed to give some account of 
each. 

The substances which we shall consider principally, as those 
most easily affected by light, will be the chloride, the iodide, 
and the bromide of silver. And it may be taken as fairly 
certain that the action of light upon silver iodide, upon silver 
bromide and upon silver chloride, will be similar in nature 
and in effects. These three salts of silver are called the silver 
haloids^ or the haloid salts of silver. Silver fluoride (AgF) is 


182 


THE CHEMISTRY OF PHOTOGRAPHY. 


also a silver haloid ; but as it is not used in photography we 
need not notice it here. 

The Effect of Light upon the Silver Haloids to produce 
Metallic Silver: — Scheele and Guthrids Theory. — The first 
man to study tlie chemical elTect of light upon any silver salt 
was Charles William Scheele, a Swedish chemist, in the year 
1777. He writes : 

“ I mixed as much of distilled water with w^ell-edulcorated 
horn-silver as would just cover this powder. The half of this 
mixture I poured into a white crystal phial, exposed it to the 
beams of the sun, and shook it several times each day; the 
other half I set by in a dark place. After having exposed the 
one mixture during the space of two weeks, I filtered the 
water standing over the l%ma cornua^ grown already black; I 
let some of this water fall by drops in a solution of silver, 
which was immediately precipitated into horn-silver. 

‘‘ I precipitated a solution of silver by sal-ammoniac ; then I 
edulcorated and dried the precipitate, and exposed it to the 
beams of the sun for the space of two weeks, when the surface 
of the white powder grew black, after which I stirred the pow^- 
der, and repeated the same several times. Hereupon I poured 
some caustic spirit of sal-ammoniac on this, in all appearance, 
black powder, and set it by for digestion. This menstruum 
dissolved a quantity of luna cornua., though some black pow- 
der remained undissolved. The powder having been washed 
was for the greater part dissolved by a pure acid of nitre, 
which by the operation acquired volatility. This solution I 
precipitated again by means of sal-ammoniac into horn-silver. 
Hence it follows that the blackness which the luna cornua 
acquires from the sun’s light is silver by reduction.” 

Converting Scheele’s terms into those of the present day, we 
can express the results which he believed he obtained by 
exposing silver chloride (horn-silver or luna cornua to the 
light by the following equation : 

AgCl + Ag 4- Cl 

Silver produces Metallic and Chlorine. 

Chlorine Silver 

* “ Traite de I’Air et du Feu.” 


183 


THE CHEMICAL ACTION OF LIGHT, ETC. 

The selection of silver chloride hy Scheele for this experi- 
ment will easily be understood, if we remember that neither 
silver hromide nor silver iodide was discovered till many years 
after. What the great Swede did was to select the most sensi- 
tive substance to light known to that age, and to endeavor to 
find out what change — if any — was produced by light in its 
chemical composition. His idea of exposing the silver chloride 
to light while in distilled water was decidedly neat. He 
reasoned that if any substance were given off or detached from 
the chloride by the action of light, that substance would be 
arrested by and become dissolved in the water. His theory 
proved correct. When he poured off the clear water and 
added it drop by drop to a “ solution of silver ” to a 

solution of silver nitrate), he obtained silver chloride (= horn- 
silver) once more. What had happened? 

In the first place, the silver chloride had suffered decompo- 
sition. Chlorine was liberated (but whether the whole of the 
chlorine or only a part of it is a question which has been 
debated ever since). How when chlorine is liberated in water, 
and is at the same time exposed to sunlight, the chlorine 
decomposes the water, and oxygen gas is set free. 

Cl + H^O = HCl + O 

Chlorine and Water produce Hydrochloric and Oxygen. 

acid 

Of course only a very small quantity of hydrochloric acid is 
formed, and this remains dissolved in the water. The water 
being then added to a solution of silver nitrate we again get a 
chemical change — a double decomposition in fact — and silver 
chloride is once more formed. 

HCl + AgNOa = AgCl + HNO3 
Hydrochloric and Silver produce Silver a^id Nitric 
Acid Nitrate Chloride Acid. 

In the second paragraph quoted above Scheele describes a con- 
firmatory experiment. Let us represent his work by equations. 

(1) He precipitates a solution of silver by sal-ammoniac. 

AgNOg + NH4CI = AgCl -f- NH4NO3 

Silver and Ammonium produce Silver and Ammonium 

Nitrate Chloride Chloride Nitrate. 


184 


THE CHEMISTRY OF PHOTOGRAPHY. 


(2) He exposes tlie precipitate of silver chloride to sunlight. 

AgCl = Ag + Cl 

Silver chloride produces Silver and Chlorine. 

(3) He soaks the black powder (a mixture of unaltered silver 
chloride with black silver) in “caustic spirit of sal-ammoniac’’ 
(= ammonia). This is able to dissolve silver chloride ; but a 
l)lack powder remains undissolved. 

(4) This black powder Scheele considers to be metallic silver, 
which has been set free bv the action of light. To prove that 
it is silver he adds it to some “pure acid of nitre” (= nitric 
acid) by which it is dissolved. 

2 Agg + 6HNO3 = 4AgN03 d- SH^O + 

Silver and Nitric Acid produce Silver Nitrate and Water and 

N2O3 

Nitrogen Trioxide. 

(5) He finally adds sal-ammoniac to the “ solution ” so 
obtained. 

AgNOg 4 - NH4CI - AgCl 4 NH4NO3 

Silver and Ammonium produce Silver and Ammonium 

Nitrate Chloride Chloride Nitrate. 

Thus the circle is completed, and Scheele finished with the 
same substance (silver chloride) as he began with. From this 
he not unnaturally concluded that the eh^ect of sunlight on 
silver chloride is to reduce it to metallic silver. 

Guthrie Sujpports Scheele’ s Theory. — The first man to at- 
tempt to demonstrate, quantitatively (f.d., by actually weighing 
the substances produced), the action of light upon chloride of 
silver was the late Professor F. Guthrie."^ His results came 
• near (but hardly siifliciently near) the actual results which 
should be obtained if silver chloride is completely decomposed 
by light into metallic silver and chlorine. He found that the 
silver chloride darkened rapidly when covered with pure and 
dry benzole (a liquid which contains no oxygen) ; and he 
writes : “ The rapid blackening which the chloride here under- 
went ]>roved the presence of oxygen to be unnecessary.” This 

Mis original paper was published in 1857. It has been republished in the British 
Jottrnal of Photography for 1885, p. 393. 


THE CHEMICAL ACTION OF LIGHT, ETC. 


185 


should be remembered in connection with tbe oxy chloride 
theory of the latent image. 

The Latent Image not .Destroyed hy Nitric Acid. — A rather 
powerful argument against the theory that the latent image 
consists of metallic silver is furnished hy the fact that the 
image is not destroyed when an exposed plate is bathed in the 
strongest nitric acid which can he used without ahecting the 
gelatine. 

Now nitric acid readily attacks and dissolves ordinary metal- 
lic silver; hut it has no effect on silver chloride. It is found 
that if silver chloride he exposed to light in a vessel containing 
nitric acid, tlie chloride blackens readily. If the black sub- 
stance formed be metallic silver one would imagine that it would 
be attacked by the nitric acid as rapidly as it was produced. 

The Latent Image Considered as an OxycJdoride^ Oxyhro- 
mide^ or Oxyiodide. — The nature of the latent image early 
attracted the attention of that ])rolific worker, Eobert Hunt. 
In his ‘‘ Eesearches on Light” * he wnltes : am inclined to 

believe that the first action of the solar ray (upon silver chlo- 
ride) is to liberate one-half the combined chlorine, which is 
very readily, moisture being present, replaced by oxygen. 

“ The absorption of oxygen, or rather its combination with 
the decomposing chloride, is proved by another very easy 
experiment. Some pure chloride of silver was arranged in a 
bent tube closed at one end, and the other end immersed in a 
bottle of distilled w^ater. In this state the chloride was 
exj)osed for many days to the action of sunshine, during which 
time it was fre(|uently shaken, for the purpose of exposing the 
whole of the powder to its influence. As the chloride dark- 
ened, the w^ater rose into the tube, and it gave a precipi- 
tate of chloride of silver on the addition of the nitrate, thus 
appearing to prove the substitution of oxygen for chlorine 
under the agency of solar radiation. It was quite evident that 
some absorption of atmospheric air had taken place. This 
explanation will also serve for the iodide, bromide, and some 
other salts of this metal.” 


Second edition, 1854, p. 80. 


186 THE CHEMISTRY OF PHOTOGRAPHY. 

Some thirty years after Hunt, the oxychloride theory* was> 
taken up by Hr. W. H. Hodgkinson, now Professor of Chem- 
istry at Woolwich. He remarks:* ‘‘As a chemist only, rea- 
soning from Abney’s experiment with silver chloride in a per- 
fectly dry state, that it undergoes no change on exposure to 
light, but only when water or substituted water is present, I 
thought it extremely likely that the colored substance was an 
oxychloride produced by the oxygen of a water molecule 
replacing chlorine in one or more molecules of silver chloride.” 

The probable chemical formula which Hodgkinson gives 
for the oxychloride is Ag^ClgO ; the corresponding formula 
for the oxybromide being Ag4pr20; and for the oxyiodide, 
Ag 4 l 30 . 

In an ordinary dry-plate, the gelatine is far from being per- 
fectly dry ; there is moisture, moreover (water vapor), in 
the air which is in contact with the surface of the plate. The 
following formula will explain the chemical change which is 
believed to take place on the supposition that oxygen forms a 
part of the latent image : 

4AgBr + HgO = Ag^BrgO + 2HBr 

Silver and Water produce Silver and Hydrobromic 

Bromide Oxybromide Acid. 

The hydrobromic acid set free probably combines with part 
of the gelatine. 

The chief objection to this “oxygen” theory is that the* 
silver haloids darken quite readily when exposed to light 
beneath liquids or gases which contain no oxygen, such a& 
benzine or hydrogen. Of course it may be said that the dark 
“ photo product ” is in such a case different from that which 
v/e get under other conditions ; but the weight of evidence is 
to the contrary. 

On this question Bothamley remarks f “ It is very difficult 
to believe that a silver oxychloride could form in presence of 
strong nitric or hydrochloric acid. It is also important to 
observe that the supposed oxychloride is not a reduction 
product of silver chloride, but a substitution product ; the 


Photo. Nc7vs, 1887, p. 370 ; and 1888, p. 531. 
+ Jotirnal Catnera Club., 1890, p. 114. 


THE CHEMICAL ACTION OF LIGHT, ETC. 


187 


quantity of chlorine and oxygen in the formula given being 
sufficient to neutralize all the combining power of the silver. 
Silver oxide is known to be readily reduced to the metallic 
state by developers ; and if we assume that this reducibility of 
the oxide is transferred to the oxychloride, which would be the 
case if the compound had the constitution represented by the 
formula given (Ag4Cl2 0 ), the formation of the oxychloride 
would certainly explain the production of an image on develop- 
ment. On the other hand, it is equally well known that silver 
oxide is very readily attacked by acids, and it is not easy to 
see how an oxychloride could retain the instability of the oxide 
in presence of reducing agents, and yet offer so great a resist- 
ance to the action of acids. If further experiments prove that 
the darkened products are really an oxychloride and an oxy- 
bromide respectively, it is not at all probable that they will 
have the constitution which has been suggested. 



CHAPTEE XYI. 


THEORY OF THE LATENT IMAGE (CONTINUED). 

^ The Sub-Salt Theory of the Latent Image . — That great 

alchemist, Albertns Magnus, who flourished in the thirteenth 
century, refers to the black hue imparted to the human skin 
when it w^as rubbed with caustic silver (silver nitrate). Then 
his successor, Eabricius, in the middle of the sixteenth century, 
tells how the miners saw the mineral called ^Miorn-silver ” 
(silver chloride) darken on its transfer from the gloomy depths 
of the mine to sunlight. A century later, Glauber and Eobert 
Boyle mention the darkening of silver compounds when long 
kept. But Schulze, in 1727, was the first who proved that 
this blackening was due to the agency of light and his experi- 
ments were conflrmed by Beccarius, of Turin (about 1750), and 
by Scheele (1777), the latter chemist being the first to attempt 
to investigate the nature of the change produced. 

Taking silver chloride as an example, we may consider the 
following facts as certain : 

(1) A short exposure to light gives a ^Hatent image,” 
invisible to the eye, but capable of being developed into a 
visible image. 

(2) A longer exj^osure to light causes the white silver chlo- 
ride to change color, first to violet and then to brown. This 
dark-colored product is probably identical with that which 
forms the latent image. We cannot see the molecules of 
altered silver chloride which form the latent image, any more 
than we can see a few small leaden shots scattered over a sack- 
ful of flour. But the continued action of light increases the 
number of the altered molecules, and they then become visible. 

(3) The exj^^sed silver chloride gives off a part or the whole 
of its chlorine, Avhich can be collected and tested in the usual 
way. 


THEORY OF THE LATENT IMAGE (CONTINUED). 189 

(4) The action of light upon silver chloride is greatly 
accelerated by the presence of some substance able to com- 
bine readily with chlorine. Such a substance is called a 

sensitizer.” 

(5) In the absence of all sensitizers or chlorine absorbents 
(as in a vacuum) pure silver chloride is not affected by light. 

(6) Exposure to the vapors of chlorine, bromine, or any 
compound which will readily part with these elements, desti'oys 
the latent image. 

The other haloid salts of silver — the bromide and the iodide 
— when exposed to light are similarly affected to the chloride. 

All these considerations help us to believe that the action of 
light upon these compounds of silver in producing a latent 
image is of a chemical and not of a physical nature. 

The point which has to be determined is this : What is the 
chemical nature of the dark-colored material produced by the 
agency of light? To this it may seem strange to say that we 
are not yet able to return a positive answer. Although the 
whole of the silver salt may appear to be converted by many 
days’ exposure and by frequent shaking into the dark material, 
yet this change is superficial only. The outside of each tiny 
particle suffers change ; but the inner and greater portion 
remains unaltered. 

The sub-salt” theory which wm have now" to describe has 
in past years received the powerful support of Dr. II. Yogel, 
and of Captain Abney. The chemistry of the sub-salts of 
silver is obscure and difficult. If there are such compounds, 
then oue of them should be the sub-oxide of silver, having the 
formula AgqO. The German chemist, Wohler, believed that 
he had obtained such a compound in 1839, but the later 
researches of dewberry, Muthmann, Yon Pfordten, Bailey, 
and Fowler go to show that Wohler must have been mistaken. 
Still, the existence of the sub-oxide is not necessary to the. 
latent image theory, wdiich declares that the effect of light 
upon silver chloride is to reduce it to the state of silver sub- 
chloride : 

2AgCl == AggCl + Cl 

Silver produces Silver and Chlorine. 

Chloride Sub-chloride 


190 


THE CHEMISTHY OF PHOTOGEAPHY. 


The iodide is similarly reduced to the suh-iodide : 

2AgI zr Ag,I + I 

Silver produces Silver and Iodine. 

Iodide Sub-iodide 

And the bromide, in its turn, is reduced to sub-bromide : 

2AgBr = AggBr + Br 

Silver produces Silver and Bromine. 

Bromide Sub-bromide 

It will be seen that this theory tits in nicely with the observed 
facts. It explains why the presence of a sensitizer is necessary 
— to absorb the haloid given off under the action of light. 
In the absence of some such absorbent, we can imagine that 
light still decomposes the silver salt ; but the liberated halogen 
at once recomhines with the sub-salt so formed, bringing it 
back to its original condition : 

AggCl -h Cl = 2AgCl 

Silver and Chlorine produce Silver 

Sub-chloride Chloride. 

That such a decomposition and recombination can take place 
was shown by Morichini, who found that when moist silver 
chloride was exposed to light in a sealed glass tube, in vacuo^ it 
rapidly blackened ; but that the white color was restored when 
the tube was kept in a dark place for a few days. 

Another — and perhaps more feasible — idea is that the light 
is unable to effect the decomposition, unaided. But when 
some substance (sensitizer) is present which exerts an attraction 
upon the haloid, then the comhined effect of light plus chem- 
ical attraction effects a separation or decomposition. 

Arguing from analogy, we might expect silver sub-chloride 
to exist ; for the metals, copper and mercury — which have 
many points of resemblance to silver — both form sub-chlo- 
rides. Thus with copper we have — 


Copper chloride CuClg 

Copper sub-chloride CuCl 

And with mercury — 

Mercury chloride {corrosive sublimate) HgClg 

Mercury sub-chloride {calo?nel) HgCl 


THEORY OF THE LATENT IMAGE (CONTINUED). 191 


The metal thallium^ discovered by Mr. Crookes in 1861, also 
forms a double series of chlorides — 


Thallium trichloride TICI3 

Thallium monochloride , , TlCl 


But all these are well-known chemical compounds, easily 
prepared. They are all white. Again, if an oxidizing agent 
be present they cannot be formed. Now we know that the 
substance which it is proposed to call silver sub-chloride is 
of a dark color, and is formed even when the silver chloride is 
exposed to light beneath the surface of such a powerful oxi- 
dizing agent as nitric acid. The action of light upon mercury 
sub-chloride is to decompose it into mercury chloride and 
metallic mercury ; and not to change it to any lower chloride : 

2HgCl = HgCl^ -f Hg 

Mercury produces Mercury and Mercury. 

Sub-chloride Chloride 

The summing-up of the whole matter is, that the evidence 
clearly proves that silver chloride^ loses chlorine on exposure 
to light ; but that it has not yet been certainly proved that the 
blackened residue is silver sub-chloride, AggCl, and nothing 
else. 

We may add that it is a fact well known to plate-makers that 
spoiled plates (coated with silver bromide in gelatine), when 
stacked in an open space exposed to light give off an odor 
which — so far as the sense of smell is concerned — is indistin- 
Miishable from that of bromine. 

The Latent Image as a I.ahe^^ — Carey Lea’s Photo 

SaltsT — The distinguished American photo-chemist, Mr. Carey 
Lea, was born at Philadelphia in 1823. English readers made 
his acquaintance in 1864, when he commenced to act as corres- 
pondent for the British Journal of I^hotography and his 
name has ever since been a “ household word ” among those 
who have studied photography mainly for the many capti- 
vating problems which it offers in chemistry and in physics. 

Of all Mr. Lea’s researches none equal in interest and 
importance the series upon the “ photo salts of silver,” and 


* And the chloride stands as a type also for the bromide and the iodide. 


19i^ THE CHEMISTRY OF PHOTOGRAPHY. 

upon “ allotropic silver,” which appeared during the years 
1887-91 in the American Journal of Science and which 
were reprinted on the English side of the Atlantic in the 
PhilosojAiical Alagazine^ and in the British Journal of 
Photography^ etc. 

In the opening paper of the series it is declared that the 
object is to show : “ (1) That chlorine, bromine, and iodine 
are capable of forming compounds with silver exhibiting varied 
and beautiful coloration, pearl-blossom, rose, purple, and 
black. That these compounds (exce]3t under the influence of 
light) possess great stability ; that they may be obtained by 
purely chemical means, and in the entire absence of light. 

(2) That of these substances the red chloride shows a 
tendency to the reproduction of colort-. 

^^(3) That these substances, formed by purely chemical 
means, constitute the actual material of the latent or invisible 
photographic image ; which material may now be obtained in 
the laboratory wdthout the aid of light and in any desired 
quantity. They also form part of the visible product result- 
ing from the action of light on the silver haloids.” 

According to this theory, when light acts upon an ordinary 
gelatine dry-plate, some of the silver bromide (AgBr) is 
reduced to the state of sub-bromide (AggBr), but it is only 
possible for the light to form a small quantity of the sub- 
bromide. The sub-bromide then enters into a molecular 
combination with the unaltered silver bromide, to form a 
purplish compound of a nature similar to what is called a. 
‘‘ lake.” Such a compound of silver sub-bromide and silver 
bromide Lea calls a photo-bromide.” Similarly, we get a 
photo-chloride ” and a photo-iodide,” when silver chloride 
and silver iodide are respectively ex]30sed to light. Collec- 
tively, they may be termed photo-salts,” as being capable of 
production by the action of light. Such compounds — unlike 
the normal sub-salts — are not attacked by strong cold nitric 
acid. 

With silver chloride Lea found that it was not possible to 
convert more than about 8 per cent, of the material into the 
sub-chloride. The black photo-chloride is easily obtained by 


THEORY OF THE LATENT IMAGE (CONTINUED). 


193 


treating reduced silver with two or three apjolications of 
sodium hypochlorite. 

The identity of the photo-salts with the material which com- 
poses the latent image is shown hy the following facts : 1 st, 
in the entire absence of light, sodium hypophosphite is able to 
affect a sensitive him of silver haloid in the same way as does 
producing a result equivalent to a latent image formed 
Ijy lights and capable of development in the same way as an 
actual impression of light. 

2ndly. That these two effects, the impression produced by 
hypophosphite and that l)y light, comport themselves to 
reagents in exactly the same way, and seem every way iden- 
tical. 

3rdly. That the image j^roduced by the action of the hypo- 
phosphite on silver chloride always gives rise to a positive on 
develoj^ment ; but on silver bromide may give rise to either a 
direct or a reversed image, both of these effects corresggonding 
exactly loith those of light. More than this, sodium hypo- 
phosphite may be made to reverse the image produced by 
light on silver bromide ; and conversely light may be made to 
reverse the action of hypophosphite. So exact a correspond- 
ence in these remarkable properties can scarcely be fortuitous. 

It is an interesting experiment to damj) the surface of a 
wood printing-block with sodium hypophospliite. Then in the 
dark-room place tlie block in contact with the surface of a 
gelatine dry-plate. On development in the ordinary way, the 
plate gives the picture which was carved upon the block. 
Alkaline solutions of milk-sugar or of grape-sugar, or a solu- 
tion of ferrous liydrate produce the same effect as sodium 
hypophosphite. The photo-products ” (this term seems better 
than ‘‘photo-salts,” as the latter would infer that the bodies 
in question have a definite chemical composition) so formed 
are affected similarly to the latent image ; being destroyed b}^ 
potassium bichromate, and decomposed by sodium hyposul- 
phite and by ammonia. 

It is to be noted that nitric acid, which attacks either silver 
alone, or silver chloride alone, or silver sub-chloride alone, has 
no effect upon the mixture (or rather molecular combination) 


194 


THE CHEMISTRY OF PHOTOGRAPHY. 


of these substances which is produced by the action of light 
upon silver chloride. Lea writes : — “ The principal action of 
light on AgCl (precipitated in ]3resence of excess of hydrochlo- 
ric acid) consists in the formation of a small quantity of sub- 
chloride^ which enters into combination with the white silver 
chloride not acted upon, forming the photo-chloride, and thus 
is able to withstand the action of strong nitric acid. At the 
same time a trace is formed, either of metallic silver or of 
uncombined sub-chloride, it is impossible to say which. After 
a certain very moderate quantity of photo-chloride is formed, 
the action of light seems to cease. 

“ The nature of the product formed by the continued action 
of light on silver chloride, seems to support the conclusion that 
the sub-chloride is combined with the whole of the normal 
chloride after the manner of lakes rather than in equivalent 
proportions.” 

The term lake ” is applied in commerce to certain colored 
compounds which consist of organic coloring matters precipi- 
tated in the presence of alumina. Ko definite chemical com- 
pound is formed, but the two substances hold so firmly together 
that they cannot be separated by rejDeated or long-continued 
washing. Probably some kind of molecular combination takes 
place. 

Summing up Carey Lea’s theory of the latent or photo- 
graphic image we note that, according to him, it consists nei- 
ther of the normal silver haloid physically modified, nor of a 
sub-salt ; but of a combination of normal salt and sub-salt. 
That the sub-salt loses in this way its weak resistance to 
reagents, and acquires stability, thus corresponding to the great 
stability of the latent image, which, though a reduction product, 
shows considerable resistance to even so powerful an oxidizer 
as nitric acid. 

The Latent Image Consists of Allotrojpie Silver Bromide — 
I.eaper^ 1891,— Wq have quoted in tabular form the different 
varieties of silver bromide, as classified by De Pitteurs. Com- 
mencing with the variety of AgBr which transmits orange 
light, we find that by the addition of energy in the form of 
heat we can steadily increase the sensitiveness of the silver 


THEORY OF THE LATENT IMAGE (CONTINUED). 195 


bromide to light, until at last we arrive at a form of AgBr 
wliicli transmits blue rays, and which is itself exquisitely sen- 
sitive to light. 

By continuing to heat the emulsion after having reached 
this point, we obtain a form of AgBr which is sensitive to red 
light, and which is at once decomposed by a developer, with- 
out having been exposed to light at all, the plate being — as we 
say — “ fogged all over.” 

Leaper argues * tliat the effect of energy in the form of light 
is similar to its effect in the form of heat. By light, the 
seventh allotropic form of AgBr shown in De Pitteurs’ table 
is converted into the eighth and last modification shown in the 
same table. The former is not affected by our developers, the 
latter is; an image can therefore be developed upon such a 
plate. 

Table of the Chemical Theories of the Latent Image. 

I. — The Latent Image consists of Metallic Silver. . | 

II. — The Latent Image an Oxy-Haloid Salt of \ Hunt, 1854. 

Silver ( Hodgkinson, 1887. 

III. — The Latent Image a “ Sub-Salt ” of Silver. . • • j 


V. — The Latent Image composed of Allotropic ) ^ 

Silver Haloid ^Reaper, l»yi 


* British Journal of Photography 1891, p. 231. 


IV. — The Latent Image a “ Lake.” 




CHAPTER XVII. 


PHYSICAL THEORIES OF THE LATENT IMAGE. 

X physical change in matter is one by wliich the chemical 
composition of the substance remains unaltered, although some 
of its physical properties, as its color, taste, etc., are changed. 

Light is able to produce physical changes in certain kinds of 
matter. When lumps of red realgar (arsenic disulphide^ 
AsgSg) are exposed to light, they crumble away to a yellow 
jDOwder. But by simply fusing this powder it is restored to the 
state of red lumps as before. Chemical analysis shows that the 
yellow powder and the red lumps have precisely the same chem- 
ical composition (AsgSg). The change of color is probably due 
to some rearrangement of or in the molecules of the realgar. 

Half a century ago, M. Moser, of Konigsberg, detailed* 
some remarkable experiments, which were re])eated and ex- 
tended by Draper,f showing that if any clear, hard surface, as 
of metal or glass, be covered with a ]3erforated screen and 
then exposed to light, an image of the screen can be subse- 
quently produced by .removing the screen and hreathing upon 
the bare glass. The water- vapor in the breath condenses most 
upon the parts which have been exposed to light. 

I. Latent Image due to Molecular Change Effected hy Imght 
— Ilardioich^ 1855. — The theory of production of a latent image 
stated by Hard wich in 1855 is illustrated by a diagram, which 
we reproduce : 




Here Fig. 1 represents a rnolecnle of ordinary silver iodide, 
the component atoms of silver and of iodine being closely asso- 
ciated, and having much chemical attraction for one another. 


* Journal of the Academy of Sciences (Paris), for 18th July, 1842, eic. 
t F hilosophical Magazine^ for September, 1840. 

X Page 109, “ Hardwich’s Chemistry,” by Taylor. 9th Ed. 


PHYSICAL THEORIES OF THE LAl'KNT IMAGE. 


197 


Fig. 2 represents the same molecule, after it has been acted 
upon hy light. The atoms are now separated so that they 
only just touch, and their mutual attraction is much weakened. 
The consequence of this weakening is that a solution (as a 
developer) wTiich has no effect upon the ordinary silver iodide, 
is able to decompose the same substance after it has been 
acted upon by light. 

The weakness of this theory lies in the fact that jpure silver 
iodide (or any other haloid salt of silver) is not affected in this 
way by light if it he exposed to light in a vacuum 

IT. The Latent Image Considered as a Ythratlon of the 
Atoms. — It is evident that any cause which weakens the force 
by which the atoms forming a molecule are held together, whil 
render that molecule more easy to be decomposed. Physicists 
and chemists agree in believing that not only are all the mole- 
cules of all matter in constant motion (this molecular motion 
w^e know as heal)., but that the atoms composing each molecule 
are themselves moving or vibrating. If this atomic vibration 
be greatly increased, it may be sufficient to cause the molecules 
to shake themselves to pieces,” and chemical decomposition 
then takes place. Thus by lieat alone we are able to decom- 
pose the red oxide of mercury into mercury and oxygen. 

HgO = Hg + o 

Oxide of produces Mercury and Oxygen 
Mercury 

But if the motion of the atoms be only increased to a certain 
extent, the effect may be that their affinity for each other will 
be just so much weakened as to allow of their decomposition 
by solutions (developers) which normally would have no effect 
up 311 them. 

Suppose we compare the two atoms which conqDOse a mole- 
cule of silver bromide (AgBr) to two balls united by a short 
piece of India-rubber. Let the normal condition of the balls 
be that of revolution round a point midway between them, 
just enough to keep the rubber stretched. Let the motion of 
the balls he now increased. The result will be that more ten- 
sion will be put on the rubber, the balls will move farther 
apart and a less force will be required to cut the connecting 


198 


THE CHEMISTRY OF PHOTOGRAPHY. 


link and so separate the balls entirely, than if they were in 
their normal state. 

When molecules of the silver haloids are exposed to light, 
the advocates of the vibratory theory ” believe that the motion 
of their atoms is increased, and that their subsequent decom- 
position by a developer is thus facilitated. 

The physical theory, in one form or another, of the latent 
image, was first advanced by Moser, to whose experiments we 
have already alluded ; it was supported by Hardwich, and 
by Dr. D. Van Monckhoven ; at one time Mr. Carey Lea was 
its principal advocate,! though his later work has led him to 
renounce it. Light is a form of energy which travels through 
space in the form of ether waves. When these waves fall 
upon the silver haloids the molecules of the latter are thrown 
into a state of unstable equilibrium, and are then readily 
affected by chemical solutions (developers) which would other- 
wise be powerless to decompose them. 

Ohjections to Physical Theories of the Latent Image . — If 
the latent or “photographic” image consists merely of the 
same substance as the unaltered silver salt, but in an abnormal 
condition as regards the position or vibration of its atoms or 
molecules, it is difficult to conceive how that abnormal position 
IS maintained. Dry-plates have been exposed, and then kept 
for several years before development, without the resulting 
image showing any lack of vigor. Is it jiossible that any 
unstable condition or vibration could have been maintained 
during so long a period ? Such a thing is not, however, im- 
possible. In our own experience as a microscopist and geolo- 
gist, we have frequently obtained thin slices of igneous rocks 
in which were cavities (visible only under the microscope) 
partly filled with some liquid, througli which a bubble of gas 
moved with rapid speed backwards and forwards. The time 
since the rocks in question consolidated, and the bubbles 
were imprisoned, must be reckoned by millions of years. Yet 
the bubbles have probably been in continuous motion ever 
since ! 


* British Jour7ial of Photography for 1863, p. 74. 
t British Journal ior 1865-66-67, numerous articles. 


PHYSICAL THEORIES OF THE LATENT IMAGE. 199 

But the case is very different with molecules of silver bro- 
mide embedded in a tough solid like gelatine ; and all analogy 
would lead us to expect that if the latent image consisted 
merely of a vibratory movement of the atoms, or of an abnor- 
mal condition of the molecules, that it would speedily disap- 
pear ; and that the plate would then be restored to its pristine 
state. 

It was formerly believed that the latent image did disappear 
when the exposed plate w^as kept for a few months. And in 
the case of an exposed daguerreotype plate, especially, this is 
found to be the case. But in all such cases the plate has suf- 
fered from the impurities always present in the atmosphere, from 
which it is found impossible to preserve the plate unless it be 
hermetically sealed up in a vacuum. Moreover, every film con- 
tains small quantities of substances which we can only consider 
as ‘Mirt” (because it is “matter in the wrong place’’) and 
these substances combine with and destroy the latent image. 

In the case of the daguerreotype, the silver plate is sensitized 
by exposing it to the vapor of iodine. IvTow there is always pres- 
ent upon the plate an excess of iodine. If the action of light 
be — as we presently hope to prove — of a chemical nature, con- 
sisting in the separation of part of the iodine contained in the 
silver iodide, then we have present in this free iodine a sul> 
stance, which is able to combine with the partly reduced silver 
iodide and so to restore it to tlie state of normal iodide. 

AggI + I = 2AgI 

Silver ajid Iodine produce Silver 

Sub-iodide Iodide. 

Thus the fading of the latent image, in the case of the 
daguerreoty]3e at all events, may be taken to be as much in 
favor of the chemical as ot the physical theory of the latent 
image. 

Starnes’ Hypothesis of the Latent Image. — Light Hiptures 
the Gelatine Casing. — A second physical hypothesis for 
explaining nature of the latent image w^as broached by Mr. II. 
S. Starnes, in He urged “that light, acting on the 


* British Jozirnal of Photography^ vol. xxx., pp. 643, 656 ; vol. xxxi., pp. 501, 712. 


200 


THE CHEMISTRY OF PHOTOGRAPHY. 


salts of silver, when held in suspension in collodion or gelatine, 
has a previous mechanical action, namely, the rapid vibration 
or expansion of the particles, which strain or burst the pro- 
tecting cells of the collodion or gelatine.’’ It is certain that 
the gelatine of an emulsion protects and wraps round the 
molecules of silver bromide ; for certain chemical solutions, 
wliich will decompose silver bromide when alone, produce no 
effect when poured upon an ordinary dry-plate. Again, if an 
emulsion of silver bromide in gelatine be exposed to light, and 
then re-melied^ it will show but the merest trace of fog on 
development. Starnes reasons, that in a dry-plate every 
molecule of silver bromide is surrounded and protected by a 
coating of gelatine. The action of light rujjtures the gelatine, 
and thus exposes the silver salt to the action of the developer. 

Objections to this theory are : (i) that by soaking an exposed 
plate in a solution of bichromate of potash the latent image is 
destroyed, hfow, how could the bichromate repair the breaches 
wdiich Starnes supposes to be made by light in the gelatine ? 

(2) According to this theory, silver bromide in collodion 
should be as sensitive to light (or more so) than the same salt 
embedded in gelatine, for collodion forms a more porous and 
delicate film than the tough gelatine. Yet the contrary is the 
case. 

(3) The latent image is destroyed by a solution containing 
bi'omine. This solution attacks the gelatine ; and, under 
Starnes’ theory, one would imagine that it would intensify — 
so to speak — the action of light, instead of destroying it. 

Electrical Theory of the Latent Image . — The idea that elec- 
tricity might have something to do with the production of the 
latent image has naturally occurred to many minds. Its possi- 
bility has of late been brought forward by Dr. T. W. Drink- 
water ; and a series of remarkable experiments by Professor 
Minchin have also been lately published which point in the 
same direction. 

An early work of great interest is Becquerel’s book, “La 
Lumiere ; ses causes et ses effets,” written about half a century 


rnal of the Camera Club^ for April, 1801. 


ago. 


I’lIYSICAL THEORIES OF THE LATENT IMAGE. 201 

The researches of the last few years have shown that elec- 
tricity has a velocity comparable, if not identical, with that of 
light (186,000 miles per second) ; and, further, that electricity 
comes from the siin in company with light. AYe know, too, 
that electricity travels in waves^ just as light is known to do. 
One of Eecquerel’s experiments was to coat two silver plates 
with chloride of silver and place them in a vessel of water, 
connecting them by wires with a delicate galvanometer.^' 
AVlien one of the plates was exposed to light, a current of 
electricity was invariably produced. Blue and ultra-blue light 
]}roduced this effect ; but not red, yellow or green. Professor 
Minchin’s experiments go much further in the same direction ; 
but the subject is very complex and much yet remains to be 
done. 

Latent Image Produced hy Pressitr^e.—lw 1885 Carey Lea 
observed t that when any hard substance was pressed upon or 
drawn over the surface of a film sensitive to light, that an 
effect was produced which — although cpiite invisible to the 
eye — could be “ developed” by the same solutions as brought 
out the latent image produced by exposing to light. 

The same effect was investigated by Captain Abney in 
1883-4.j: If we write upon the surface of dry-plates (of 

course in ruby light only) with the rounded end of a glass 
rod, the letters formed will stand out in black when a devel- 
oper is applied to the plate Any hard material may be used 
instead of glass, and the effect is transmitted through paper, 
if a sheet of that substance be interposed between the rod and 
the film. The image so formed behaves similarly to the latent 
image produced by light (being destroyed by potassium bichro- 
mate) and is probably identical with it. It appears to lie at 
the bottom of the film, next the glass. 

The recent investigations made by the Belgian chemist. 
Professor Spring, show that chemical changes are produced 
in many mixtures when they are submitted to great pressure. 
And when a sensitive film is submitted to shearing stress” 


*An instrument for detecting the occurrence of an electric current. 
^British Jotirnal of Photography^ vol. xiii,, p. 84. 

X Journal of the Photographic Society of Great Britain. 


202 


THE CHEMISTKY OF PHOTOGKAPHY. 


or pressure, iHs probable that a decomposition of tbe silver 
haloid is brought about, which is revealed when the film is 
submitted to the action of a developing solution. 

Table of Physical Theories concerning the Latent Image. 

I. — The Latent Image due to a Molecular Altera- ) Moser, 1842. 

tion in the Silver Haloid j Hardwich, 1855. 

II. — The Latent Image due to Vibration of 1 ^ j 

Atoms parey Lea, 1865. 

III. — The Latent Image due to Rupture of Gelatine ;j^g 33 

IV. — Electrical Theory of the Latent Image Drinkwater, 1888. 

V. — A Latent Image Can Be Produced By Pres- ) Carey Lea, 1866. 

sure ) Abney, 1883. 

Summary of the Whole Question of the Latent Image. 

The facts point conclusively in the direction of some chem- 
ical change. Probably the hypothesis of Carey Lea, that the 
latent image is a molecular combination of the nature of a 
“ lake,” accounts better for the observed facts than any other 
theory. 

Some Important Papers on the I^ature of the Latent 

Image. 

Photographic News: 

1887. — Hodghinson^ Dr. W. E . — The Chemistry of the 
Latent Image; p. 370. 

1888. — Ilodghinson, Dr. W. E. — Lowest Stages of Com- 
bination of Silver ; p. 531. 

. Drinkwater^ Dr. T. W . — Some Notes on the Nature 

of the Latent Image ; p. 390. 

1890. — iSpiller, J . — The Chemical Phenomena of Light 
(Dr. Percy and G. Shaw) ; p. 256. 

Meldola^ Professor E . — The Photographic Image ; 

pp. 557, 580, 599. 

British Journal of Photography: 

1887. — Lea^ M. Carey. — Plioto-Salts of Silver; pp. 330, 
345, 472, 486, 522. 

1888. — Gifford.^ II. J . — Notes on the Nature of the Latent 
Image ; p. 403. 


PHYSICAL THEORIES OF THE LATENT IMAGE. 


20a 


18S9. — Braham^ P. — Light, its Chemical Action; p. 92, 

» Wiggin, J. CL — The Chemistry of Photography ; p. 

348. 

Bedding^ T . — Continiiating Action of Light ; p. G19. 

— Tlie Negative Image ; pp. 6«4, 716, 732, 

776. 

Lea^ M. Cai^ey . — Allotropic Forms of Silver; pp. 

444, 461, 478, 494, 575, 621, 638, 814. 

1890. IlitcliGOck^ P . — Action of Liglit on Silver Chloride ; 
pp. 8, 66, 188, 222, 301. 

Botliamley^ C. II . — The Latent Photographic Image ; 

pp. 235, 243, 248. 

Brebne7\ II — Nature of the Invisible Image ; pp. 

487, 551, 617, 631, 649, 682. 

1891. — Lea., M. Carey . — Allotropic Silver; pp. 229, 262, 
627, 726. 

Leaper^ C. J . — New Tlieory of the Developable 

Image ; p. 231. 

Sutton’s Photographic Notes : 

1856. — Franklaiid., Dr. E. — On the Chemical Changes 
occurring in Photography ; Ycl. L, Nos. 5, 6, and 7. 

Chemical Society’s Quarterly Journal: 

1857. — Guthrie^ Fred . — On the Action of Light upon Chlo- 
ride of Silver ; Yol. X., Part I. (Reprinted in British 
Journal of Photography for 1885, p. 393.) 



CIIAPTEK XYIII. 


THE CHEMISTRY OF DEVELOPMENT.— (I.) DAGUER- 
REOTYPE PROCESS. 

What is Development? By “development,” in photog- 
raphy, we making plainly visible oi any image 

which was previously invisible, or at all events scarcely discern- 
ible. 

When a sensitive surface, as that of a dry-plate, is exposed 
to light within the camera, an image, called the latent, invisi- 
ble, or photographic image, is impressed upon it. This image 
is not visible upon the surface of the plate. But by applying 
to the plate certain solutions, called “ developers,” the image is 
made visible. 

It is plain that a developer must be some substance which 
acts differently upon the parts of the sensitive surface which 
have been affected by light, as compared with those which the 
light has not affected. By this differential action the contrast 
between the exposed and unexposed parts increased \ and 
the latent image then becomes the visible or developed image. 

The First Man who Developed a Plate. — The first man who 
has a real claim to be considered a “ photographer” was Joseph 
Nicepliore Niepce, of Chalon-sur-Saone, in France. He was 
the first to take a picture in the camera ; the first to develop a 
plate ; and the first to secure a permanent j)hotograph. He 
was about forty-eiglit years of age wdien he commenced (in 
1813) to work at the problem of securing pictures by the 
agency of light. By the year 1827 he had certainly achieved 
considerable success, for in that year he paid a visit to his 
brother Claude (then residing at Kew, in England), bringing 
with him several specimens of his work. He did not divulge 
his method, but some of his “ photographs,” which he presented 
to certain of Ids friends in England, are now in the British 
Museum, and are very creditable indeed. He labored in vain 
to ]>erfect his discovery ; entered into partnership with Da- 


THE CHEMISTRY OF DEVELOPMENT, ETC. 


205 


gnerre in 1829 ; and died, a disappointed man, in 1833, aged 
sixty-eight. 

How Niepce Developed his Plates . — Tsiepce coated metal 
plates with bitumen, dissolved in oil of lavender. By an 
exposure in the camera for several hours, a latent image was 
impressed on these plates. But the process was too slow for 
camera work, and most of Niepce’s specimens were procured 
by contact-printing — an engraving (rendered transparent by 
varnishing) being laid upon the bitumenized plates, and the 
whole then exposed to sunlight. 

The effect of sunlight is to oxidize the bitumen ; oxygen, 
from the air, combining with the bitumen to form complex 
organic compounds whose precise chemical nature it is impos- 
sible to determine. Such oxidized bitumen is harder, and is 
insoluble in liquids, such as oil of lavender and petroleum, 
which readily dissolve bitumen which has not been exposed to 
light. It is only necessary, therefore, to soak or wash the 
exposed jdate with some bitumen solvent, in order to remove the 
unacted-on bitumen, wdiile the insoluble remains, forming the 
high-lights ” of the now visible picture. 

A letter from Niepce to Daguerre is in existence, bearing 
the date 5th December, 1829, in which the phenomena of 
development are graphically described : — 

‘‘ The plate (which had been coated with bitumen) may be 
immediately submitted to the action of light in the focus of 
the camera. But even after having been thus exposed a 
length of time sufficient for receiving the impressions of exter- 
nal objects, nothing is apjDarent to show that these impressions 
exist. The forms of the future picture remain still invisible. 
The next operation then is to disengage the shrouded imagery, 
and this is accomplished by a solvent, consisting of one part by 
volume of essential oil of lavender and ten of oil of white petro- 
leum. Into this liquid the exposed tablet is plunged, and tlie 
operator, observing it by reflected light, begins to perceive the 
images of the objects to which it had been exposed gradually 
unfolding their forms. The plate is then lifted out, allowed to 
drain, and well washed with water.” 

Many millions of plates have been developed since the days 


206 


TPIE CHEMISTRY OF PHOTOGRAPHY. 


of Niepce ; but probably no man has witnessed the modern 
miracle” with such joy, wonder, and surprise as he whose 
eyes first saw the invisible image made visible. 

The method of development necessary in heliography, or 
Niepceotype, is a physical method. It depends on a differ- 
ence in solubility between two substances — oxidized and un- 
oxidized bitumen. 

Daguerre^ s Method of Development. — Once an idea has 
been communicated, a principle established, or a fact demon- 
strated, the thing becomes familiar and more discoveries are 
sure to follow. Daguerre repeated the work of Niepce, and 
so the development of a latent image became a familiar idea 
to him ; but he failed to attain the necessary rapidity which 
he rightly recognized as indispensable to commercial success 
in photography, and so he experimented in every direction, 
trying to secure this indispensable factor. 

Daguerre appears from his early correspondence with 
Niepce, about 1828, to have always had an inclination for the 
use of iodine in his photographic experiments. Niepce had 
used the same substance in conjunction with metal plates, but 
without success. After the death of Niepce, in 1 833, Daguerre 
continued to work at the problem. 

The exact date cannot be fixed, but it was probably in or 
about 1836 that a “happy accident” is said to have rewarded 
the French scene-painter for all his toil and trouble. 

It appears that Daguerre discovered that silver iodide, 
formed and exposed upon a plate of silver, was sensitive to 
light. In this case the metallic silver at the back of the silver 
iodide acts as a sensitizer, absorbing and chemically combining 
with the iodine liberated by light. It has even been shown 
by Carey Les. that silver iodide can act as its own sensitizer. 
"VYe may perhaps suppose that a higher iodide of silver exists, 
in which case the following equation would represent the fate 
of the iodine liberated by light : 

Agl +21 = Agla 

Silver Iodide and Iodine prodtice Silver Ter-Iodide. 

Be this as it may, Daguerre found that by a prolonged 


THE CHEMISTRY OF DEVELOPMENT, ETC. 


207 


exposure in the camera he obtained a faint printed-out image 
of objects in bright sunshine, in about two or three hours. This 
was no more rapid than poor Niepce’s work with bitumenized 
plates, or than the similar results which Fox Talbot was at the 
same time (1835-39) obtaining in England upon paper coated 
with silver chloride. But fortune favored Daguerre. One 
daj he removed from his camera an iodized silver plate which, 
although it had been exposed in the usual way, showed no 
visible sign of an image. It was, as we should say, greatly 
under-exposed.” This plate Daguerre put away in the cupboard 
in which he kept his chemicals. Going to this cupboard the 
next day, Daguerre was surprised, and doubtless much pleased, 
to see that the face of the iodized silver plate was no longer 
blank, but that it bore a good image of the objects towards 
which the lens of the camera in which it ’was exposed had 
been directed. The plate had, in fact, been developed during 
the night. But how, and by what ? A study of the contents 
of the cupboard revealed an open dish of mercury, upon or 
close to which the under-exposed plate had been laid. 

Further experiments were quickly made; and it was found 
that mercury va]ior possessed the marvelous power of bringing 
out or developing the latent image on an iodized silver plate 
which had received only from ten to thirty minutes’ exposure 
within the camera. By warming the mercury in a small iron 
pot, over which the exposed silver plate was suspended, 
iodized side downwards, the speed of development w^as 
increased so that instead of requiring all night ” (as in 
Daguerre’s cupboard) the operation w^as completed in a few 
minutes. 

The development of a daguerreotype belongs to \\\q physical 
class. We can conceive of the exposed plate over the warm 
mercury as being subjected to a bombardment of millions of 
molecules of mercury all over its surface. The portions of the 
plate affected by light are able to combine or amalgamate with 
this mercury ; but from the unaffected parts the mercury 
molecules bounce back again. The latent image is thus built up 
or developed by the accretion of mercury molecules. Professor 
Meklola has "veil compared the action to the effect of a sand- 


208 


THE CHEMISTRY OF PHOTOGRAPHY. 


blast upon a sheet of glass on which a design has been painted 
in gum. The particles of sand which strike the gummy parts 
adhere to them, and so a design is ‘‘ developed ’’ in sand parti- 
cles. As to the reason why the molecules of mercury combine 
only with the portions of the plate which have been affected by 
light, we know little or nothing. The action may be chem- 
ical — some definite compound being formed between the mer- 
cury and the “ photo-salt ” or reduction product ; or (more 
probably) it may be merely physical, the mercury being able 
to amalgamate with the sub-iodide of silver, but not with the 
normal silver iodide. 

It is possible to (temporarily) develop a daguerreotype plate 
by simply breathing upon it. The photo-reduction product 
attracts, or rather combines, with the moisture, just as it attracts 
the mercury. 



CriAPTEK XIX. 


CHEMISTRY OF DEVELOPING PROCESSES.— (II. ; : 
CALOTYPE AND WET COLLODION. 

Chemistinj of Calotyjpe Development . — It must, we think, be 
granted, that if the original processes for photography pub- 
lished (1) by Daguerre, and (2) by Fox Talbot in the year 1829, 
be compared, tlie advantage lies on the side of the Frenchman ; 
and this because he had discovered a procei|^ of development., 
while Talbot’s photogenic drawings ” w^ere necessarily 
out in the camera. 

But in September, 1840, Talbot discovered a method of 
development which placed his process practically on a level 
with that of his foreign rival. The same process of develop- 
ment was discovered, inde23endently, in the same year by an 
English clergyman, the Bev. J. B. Beade. 

Talbot named his new method the Calotype^ and he patented 
it early in 1841.^^ Ills sensitive sur-^ace consisted of sheets of 
paper coated with silver iodide which, when it was desired to 
prepare them for use, were brushed over with a mixture of 
silver nitrate, gallic acid, and acetic acid. To this mixture the 
name of ^‘gallo-nitrate of silver” was applied. After exposure 
in the camera, the image w^as either invisible or very faint ; it 
was then brought out, strengthened, or “ developed ” by pour- 
ing over it more of the ‘‘ gallo-nitrate of silver” solution, to 
which some alcohol w^as usually added in order to cause it to 
tiow freely over the plate. 

Here we have a developer containing three ingredients. Let 
us consider its chemical action, and the use of each ingredient. 

Pure gallic acid was first obtained by Scheele in 1786. Its 
molecule contains eighteen atoms, C.^IT(.Og. It eagerly com- 
bines with oxygen, and with the halogens ; and is therefore 
styled a “powerful reducing agent.” 

* The exact date was February 8, 1841. This was the third British patent taken out 
in connection with photography. 


210 


THE CHEMISTRY OF PHOTOGRAPHY. 


The silver nitrate, to begin with, acts as a sensitizer, com- 
bining with the iodine which is given off under the agency of 
light. Let us first represent the decomposition of the iodide 
of silver when lights acts upon the sensitive plate : 

Agl = Agj + I 

Silver Iodide produces Silver Sub-iodide and Iodine. 

The silver nitrate then attracts and combines with the 
liberated iodine : 


6AgN03 + 

61 + 

3H3O 

=: 5 AgI 

+ 6HNO3 

Silver and 

Iodine and 

Water 

produce Silver 

and Nitric 

Nitrate 



Iodide 

Acid 




+ 

AgI03 




and 

Silver lodate. 


The latent image is thus formed of silver sub-iodide, Aggl. 
Now this silver sub-iodide has a greater attraction for, or is 
better able to combine with, nascent or freshly liberated silver, 
than the silver iodide which constitutes the surface of the film 
where it has not been affected by light. 

What has to be done then is to produce metallic silver upon 
the surface of the film. There must also be a layer of water 
upon the surface to hold the chemicals in solution, and to 
allow the attracted atoms of silver to move freely towards the 
attracting molecules of silver sub-iodide. 

By applying to the surface of the film a mixture of silver 
nitrate and gallic acid only, we get a copious, indeed too copious, 
jiroduction of metallic silver. The result of this would be a 
deposit of silver all over the plate, by which it would be 
“ fogged ” and spoilt. Here comes in the use of the acetic 
acid. This substance acts as a restrainer^ retarding the pre- 
cipitation of the silver, and giving time for the sub-iodide to 
exercise its attractive infiuence, so allowing this sub-salt to 
draw to itself all the silver atoms as rapidly as they are pro- 
duced. 

C,H.,(H0)2.C00H + 2AgN03 + HgO = Ag^ 

Gallic Acid and Silver Nitrate and Water produce Silver 

+ 2HNO3 [C6H2(H0)3.C00H + 0] 

and Nitric Acid and Oxidized Gallic Acid. 


THE CHEMISTRY OF DEVELOPMENT, ETC. 


211 


Chemical Action of Develojpment in the W et- Collodion 
Pi 'ocess . — In the wet- collodion process, as published by F. S. 
Archer, in 1851, the developer was composed of 

Water 1 ounce 

Acetic acid 1 drachm 

Pyrogallic acid 3 grains 

Archer claimed that the great power of pyrogallic acid in 
bringing out the latent image was first made known by me in 
a short description in the May number of The Chemist, for 
1850.” Pyrogallic acid was discovered by Braconnot, in 1831 ; 
and Professor Meldola writes that its use as a photographic 
developer was suggested in 1851, by Liebig and Eegnaiilt.” 
He seems, therefore, to have overlooked the claims of Archer. 
It is obtained by strongly heating gallic acid, when carbonic 
acid is given off : 

= C,H,{llO), + CO, 

Gallic Acid produce Pyrogallic Acid and Carbonic Acid Gas. 

Archer’s developer, as given above, appears to contain only 
two ingredients, pyro and acetic acid, but a third and very 
necessary part consisted of the solution of nitrate of silver 
with which the surface of the wet-collodion plate was covered, 
both during exposure and development, and which it derived 
from the bath of silver nitrate into which it was plunged just 
before exposure. 

This silver nitrate was reduced by the pyrogallic acid, 
metallic silver being set free, which immediately attached 
itself to the sub-iodide of silver which constituted the latent 
image : 

H,0 + C6H3(H0)3 + 2AgN03 = Ag, + 

Water and Pyrogallic Acid and Silver Nitrate produce Silver and 

2 HNO 3 -f [C 6 H 3 (H 0)3 + 0 J 
Nitric Acid and Oxidized Pyro. 

The precise chemical nature of the com]30und resulting from 
the oxidation of the pyrogallic acid is not certainly known. 
It is of a dark color, and is possibly allied to ulmic or humic 
acids. 


212 


THE CHEMISTRY OF PHOTOGRAPHY. 


Develop7nent of Wet-Collodion Plates with Ferroiis Sul- 
jjhate. — Ferrous sulphate (formerly called protosulphate of 
iron) was introduced as a developer by Robert Hunt in 1844,, 
for calotype pictures. It was also found to answer extremely 
well for collodion work, and was generally known as the iron 
developer.” It was usually mixed in the proportion of 20 
grains of ferrous sulphate, and 20 minims of acetic acid, with 
one ounce of water. The wet-collodion plate had a solution 
of silver nitrate clinging to its surface. 

When such a developer was poured upon the exposed plate, 
the following chemical reaction first took place : 

GAgNOg -h 6 FeS 04 = SAgg + 2 Fe 2 (S 04 )s 

Silver Nitrate and Ferrous Sulphate produce Silver and Ferric Sulphate 

+ Fea(NOg)g 

and Ferric Nitrate. 

The nascent metallic silver is attracted, as rapidly as it is 
produced (the acetic acid prevents it being produced too rap- 
idly), by the sub-iodide of silver which constitutes the latent 
image. Tins attraction is of ^.johysical nature ; and so, although 
the silver is liberated by a chemical reaction, yet the actual 
process of development belongs to the physical class of phe- 
nomena. 

Chemical and Physical Pestrainers, — The addition of an 
acid to the developers we have described slows their action 
considerably. Inorganic acids, as nitric or sulphuric, act too 
powerfully ; and of the organic acids, acetic acid seems to 
accomplish its task with the greatest regularity. It is probable 
that the acid forms a molecular combination with the silver salt 
which has not been acted upon by light ; and this compound 
does not attract silver, which is thus deposited upon the latent 
image only. 

But if we thicken the developer, as by using some colloid 
substance, such as gelatine, we restrain the movement of the 
silver molecules, and again we give time for the silver sub-salt 
(which constitutes the latent image) to exercise its sujierior 
power of attraction. Thus by adding glycerine (or a strong 
solution of gelatine) to the developer the acetic acid my be dis- 


213 


THE CHEMISTRY OF DEVELOPMENT, ETC. 

pensed widi. The latter is a chemical, the two former are 
jjhysical restrainers. A developer on this principle w^as rec- 
ommended by Mr. Carey Lea in 1875,^ under the title of the 
“ ferro-gelatine,” ‘^collo,” or “glycocoll” developer. 

Physical Development Acts Externally. — All the methods 
of development which w-e have so far described may be caljed 
pjJnysical methods. Molecules, either of mercury (in the da- 
guerreotype process) or of silver (calotype, collodion, etc., proc- 
esses), are brought into contact wdth a sensitive surface upon 
which a latent image has been produced by the action of light. 
The metallic molecules attach themselves to, or deposit them- 
selves upon, the latent image in proportion to the intensity of 
that image. f The action is of the nature of crystalline growth; 
and reminds one strongly of the methods of electro-deposition by 
wdiich gilding or plating is performed. The supply of silver 
comes from the silver nitrate wdth wdiich the plate is bathed, 
and not from the silver iodide in the film. With an ordinary 
wet-collodion plate this can be proved by w^ashing the exposed 
plate in distilled water before applying the ordinary develop- 
ing solution ; it will then be found impossible to develop an 
image of any density ; but by pouring off the developer and 
adding to it a silver nitrate solution, a satisfactory image will 
at once grow up when the developer is once more poured upon 
the plate. Or the latent image can be developed in mercury 
if tlie exposed and washed plate be treated with a solution of 
pyrogallic acid and mercurous nitrate. (Carey Lea.) 

The ridges formed by the deposit of silver can actually be 
seen upon a developed wet-collodion plate; and they visibly 
obstruct the flow of developer when it is repeatedly poured 
over the surface of the plate. Moreover, the developed image 
can be destroyed by bathing the plate in dilute nitric acid, 
which attacks and dissolves the metallic silver at the surface. 

Thus physical development is a process which acts from the 
ontside, piling up an image which is raised ahove the surface 
of the film. 


* See British Journal of Photography for 13th of August, 1875. 

t The varying intensity of the latent image being in turn due to the varying intensity 
of the light by which it was produced. 


CHAPTER XX. 


^HE CHEMISTRY OF ALKALINE DEVELOPMENT. 

Of the thousands who daily mix their pyro, ammonia, and 
bromide for use in development, how many, we wonder, give 
a thought to the “ fathers of photography ” who racked their 
brains to discover for us a wonder-working liquid, the applica- 
tion of which to a dry- plate should evolve with force and rapid- 
ity the picture drawn upon the plate by the lens ? 

Looking back for the origin of alkaline development, there 
is no doubt but that the idea was due to H. T. Anthony, of 
Xew York; and that it was extended by Leahy, of Dublin; 
Glover, of Liverpool ; and (above all) by Major Russell. 

The Photograjpliic News for August 8, 1862, contains a let- 
ter from Mr. E. F". Thompson, of 2 Wall Street, New Y^ork, in 
which he writes : Tlie problem of instantaneous dry-plates is 

about solved by H. T. Anthony, Esq., of this city. His dis- 
covery consists in subjecting a tannin dry-plate to the fumes of 
weak ammonia for a few seconds, and exposing it within one 
day after fuming. These plates are extremely sensitive, two 
seconds’ exposure being sufficient with small diaphragm, and 
instantaneous with full opening of Llarrison’s stereo portrait 
lens. The development is conducted cold in the ordinary 
manner.” 

It is probable that Mr. Anthony was induced to try the effect 
of ammonia fuming upon collodion dry-plates by the success 
which had attended his plan of treating albumenized silvered 
paper in the same way ; a plan which it appears he practised 
as early as 1860.’^' 

Another American worker, Mr. E. Borda, published certain 
experiments on rapid dry-plates in the American Journal of 
PhotofjragJlJ for lS62.f 


* See letter by Col. Sellers in British Journal of Photography for January 1, 18C3. 
t Referred to by Col. Sellers in British Jo7irnal for Aug’ust 15, 1862. 


THE CHEMISTRY OF DEVELOPMENT, ETC. 215 

He states that having tried the plan of fuming tannin dry- 
plates before exposure as suggested to him by Mr. H. T. 
Anthony, lie had gone further, and found that fuming after 
exposure, but before development, answered equally well. 

The first British experimenter to repeat Anthony’s and 
Borda’s experiments was John Glover, of Liverpool, whose 
article on The Dry Development of Dry-Plates ” appeared 
in the British Jouymal of Pliotography^ for October 1, 1862. 
The method was carried a step further by T. M. Leahy, of 
Dublin, who — writing in the Photographic P~ews^ for Novem- 
ber 7, 1862 — says : In some experiments with the honey and 

tannin process in which I tried fuming with ammonia as an 
accelerator, I remarked that, wlien the plate was washed after' 
the fuming, the image came out very distinctly ; it struck me 
that the ammoniacal vapor might have become, in some man- 
ner, fixed on the plate, and that, on the application of the 
washing-water, it dissolved and acted as a developer. Follow- 
ing up this idea, I gave a plate a very short exposure in the 
camera, and immersed it in a very weak solution of ammonia ; 
almost immediately the picture began to appear, and continued 
to come out until nearly all the details were visible. I then 
washed it well and applied the pyrogallic acid and silver, 
which rapidly completed the development of the picture, with- 
out the least sign of fogging or stain of any kind. 

“ This development of the latent image could not have 
resulted from any free nitrate being left in the him, as I not 
only wash it thoroughly after sensitizing, but also pour a 3- 
grain solution of chloride of sodium two or three times over 
it, when I again wash and pour on the tannin and honey solu- 
tion. The use of the ammonia in tlie liquid form, 1 think, 
has one great advantage over the fuming, it acts equally, and 
the picture being washed before applying the pyrogallic acid 
and silver, no deposit (such as sometimes occurs when the 
fuming is carried to any extent) can take place.” 

Step by step the method of alkaline development advanced : 
Anthony uses the fumes of ammonia ; Leahy applies the same 
alkali dissolved in water. But it was reserved for Major Bus- 
sell to perfect the method. In the British Journal of Phot ng~ 


216 


THE CHEMISTRY OF PHOTOGRAPHY. 


raj)hy, for 15tli of November, 1862, Riisseii writes: ‘^Having 
read the accounts from America of fuming dry-plates with 
ammonia, I next set about examining the capabilities of this 
agent, and during the last six weeks have made a great number 
of experiments with, to say the least, very promising results. 
. . . . Thinking that the developing action of the fumes of 
amiiionia must be due to their action on the tannin, the first 
thing I did was to try the effect of mixing a small quantity of 
ammonia with a solution of pyrogallic acid, which is much 
more unstable. The liquid showed no immediate effect, but 
changed color slowly in much the same manner as if nitrate 
of silver and acid had been added. On mixing the pyrogallic 
acid and ammonia, and immediately pouring it on an exposed 
plate, its developing action is very energetic, not only bringing 
out the image after very short exposure, but even in some 
cases producing a considerable although insufficient amount of 
intensity, which can very easily be increased to any extent by 
redeveloping''^ with pyrogallic acid and silver. Ammonia will 
develop a picture by its action on tannin if the exposure has 
been long enough ; but it must be much longer than is required 
when jiyrogallic acid is used in the same way. 

The 2 ^^’iiicipal precautions necessary are : 1st. That too 

much ammonia be not used ; one drop of the strongest solu- 
tion usually sold in four ounces of w^ater generally seems to be 
sufficient, with a few drops of strong alcoholic solution of pyro- 
gallic acid added to the portion to be used. There appears to 
be considerable latitude in the ^^roportion of ammonia ; but if 
too much is used the liquid becomes strongly colored very 
quickly, the high lights start out at once with some intensity, 
but the other portions of the plate show nothing but brown 
discoloration. 2d. That the alkaline and acid developments 
be kept quite sejiarate, the plate being thoroughly washed 
under a stream of water after the former. If this is neglected 
the jDicture will be entirely spoiled. 

Wlien these precautions are observed this method appears 
to be easy and certain, and the picture is very bright, clear, 


* We should now say “ intensifying^.” — W. J. II. 


THE CHEMISTRY OF DEVELOPMENT, ETC. 


217 


and free from loose deposit, nmcli more so than when the 
ordinary plan is adopted with an under-exposed jdate. The 
image is entirely in the him, and shows little or no dullness of 
surface on any part, even when the exposure has been as short 
as possible to produce a tolerable picture. 

These facts appear to throw doubt on the correctness of 
some of the commonly received opinions as to the nature of 
the developing action. The effect does not depend on the 
presence of nitrate of silver, for pyrogallic acid and ammonia 
will succeed on a plate which has been immersed for some 
time in a very strong solution of salt, after the latter has been 
removed by copious washing and long soaking. 

It is hardly safe to venture an opinion as to the theory of 
the matter in the present state of our knowledge, but it seems 
to me that the decomj)Osition of pyrogallic acid darkens the 
bromide or other insoluble salts of silver which are in contact 
with the impressed iodide. If this be so, it may account for 
the strongly accelerating effect of bromide (of silver) on dry- 
plates without nitrate, when used in a much larger proportion 
than would be advantageous on wet-plates from which the 
nitrate is removed.” 

One more step! Hussell mixes the ammonia loitli the 2 ) ijro^ 
and finds the mixture brings out an image capitally. 

A year later. Major Kussell describes’'^ the development of 
bromised collodion plates with a solution of carbonate of am- 
monia and pyro. With bicarbonate of soda and pyro the plate 
was quickly fogged. 

It was seen that the new method of alkaline development 
was very promising ; but it was soon found to be most suc- 
cessful with plates containing hromide of silver. The devel- 
oper consisting of ]3yrogallic acid plus an alkali was, however, 
very frequently found to fog tlie plates. It was again reserved 
for Kussell to discover f that the remedy for this fogging was 
the addition of a soluble bromide. He writes : 

The most advantageous way of doing this seems to be to 
moisten tlie film just before developing with a weak solution 


British Journal of Photography for January 1, 1663. 
t British Journal of Photography^ for 15th June and 1st July, 1864. 


218 


THE CHEMISTRY OF PHOTOGRAPHY. 


of bromide, or to mix a little of tlie solution with the alkaline 
developer, it does not much matter which, provided a suitable 
quantity of bromide is used in either case.” 

Major E-ussell was (very properly) proud of this discovery, 
which removed a great difficulty from the path of the early 
experimenters with alkaline development. Twenty-three years 
later he wrote in the British Journal Almanac ” * an article on 
‘ffilow the Eestraining Action of Bromide was Discovered.” 
This article is very short, and we may quote it in full, as a 
tribute to the man who first put together the three ingredients 
of our alkaline developer : 

“ On finding out that great sensitiveness could be obtained 
on dry-plates prepared with bromide of silver in collodion, 
and experimenting with a view to discover the conditions most 
favorable to sensitiveness, at first it seemed as if the more 
washed the film the more sensitive. The plan was then tried 
of leaving the plates, after sensitizing, to soak in water for 
twenty-four hours. 

The plates thus treated, to my surprise, always fogged 
badly. On consideration, it seemed plain enough that the fog- 
ging must be caused by the too complete removal from the 
film of soluble bromide which had escaped decomposition by 
the nitrate bath. 

“ A few trials showed that this was so, and that soluble bro- 
mide is a restrainer for bromide of silver, treated with an alka- 
line develo23er.” 

The dry -plates” referred to in these experiments of 1862-3 
were prepared by giving the plate a coating of collodion con- 
taining either a soluble bromide plus an iodide, or a soluble 
bromide alone. Such 2 :>lates were sensitized by immersion in a 
bath of silver nitrate, the result being the formation of silver 
bromide (or silver bromide ])lus silver iodide) in the film. They 
were then washed (to remove the excess of silver nitrate), 
bowed over with a solution of tannin, and finally dried. 


* For 1887, page 240. 


CHAPTER XXI. 


CHEMISTRY OF DEVELOPMENT.— (111.) BROMIDE 
OF SILVER IN GELATINE. 

The First Gelatine Emulsion Dry -Plates Developed hy 
Maddox in 1871. — When Dr. Maddox introduced the now 
universally practised gelatine dry-plate process in he 

found that he was able to develop an image upon them with 
pyrogallic acid alone, using a solution of 4 grains of pyro to 
the ounce of water. And this leads us to notice that it is the 
pyrogallic acid which is the real or principal ingredient in the 
developer. Pyro can develop an image by itself ; the ammo- 
nia serves merely as an accelerator, and the bromide as a 
restrainer. 

Maddox developed a thin picture with pyro alone ; and then 
washed the plates and intensified them with silver. He 
attempted to use ammonia with the pyro, but the plates then 
fogged. He apparently did not think of the necessity for using 
a bromide in addition, as recommended by Russell. 

Which is the Best Developer f — The first gelatine emulsion 
dry-plates ever sold commercially were made by J. Burgess, 
of London, in 18Te3; they were developed with ‘^alkaline 
pyro”; and the same developer was recommended by Mr. 
Kenneth (also of London), who strove hard to introduce simi- 
lar plates into general use between 1874 and 1877. 

Thanks to the discoveries of Bennett, Mansfield, and others 
in 1878-9, as to the wonderful rapidity to be obtamed in the 
gelatine emulsion by the use of heat in its preparation, gela- 
tine dry-plates came fairly to the front in 1879, and they 
ousted collodion from tlie supremacy which it had enjoyed for 
nearly thirty years. 


* British Journal of Photography for September 8, 1871. 


220 


THE CHEMISTKY OF PHOTOGRAPHY. 


During the early years of the dry-plate era — 1879-85, while 
the ordinary alkaline developer (consisting of pyro with am- 
monia and a bromide) was in great favor in England, workers 
on the Continent preferred ferrous oxalate ; while in America 
one of the fixed alkalies — either carbonate of soda or carbonate 
of potash — was preferred to ammonia. During recent years, 
however, the claims of pyro over ferrous oxalate have been 
very generally admitted. With plates of inferior quality (and 
the dry~23lates made on the Continent were certainly not equal 
to English plates) ferrous oxalate gives a brighter picture, but 
it does not permit the latitude of ex|30sure which is the most 
valued feature of 2 >yrogallic acid. 

But other developers have risen up to dispute the field with 
pyro. First we had hydroquinone, then eikonogen, and lastly 
para-amidophenol. Their chemical action in the developer is 
similar to that of pyro. But it is to be doubted if any one of 
them is quite so good for all-round work as pyro. We once 
(perha]3S rather rashly) made the assertion that “ the man who 
is to discover a better developer than pyro-ammonia is not 
born yet ” — but nothing has been done so far to disprove this 
statement. 

Chemistry of the Development of Gelatine Dry-Plates . — 
The sensitive surface of the gelatine dry j^lates, or films, of 
which millions are now used annually, consists of molecules of 
silver bromide embedded in gelatine. When dry, the coating 
of “ gelatino-bromide of silver ” forms an extremely thin layer, 
adhering to the glass or celluloid ; but when wetted by the 
developer the gelatine swells u^o and forms a layer about the 
one-thirtieth of an inch in thickness. When exposed within 
the camera a latent image is formed upon the surface of the 
film ; and for our present |3urpose we will consider this invisi- 
ble image as consisting of silver sub-bromide, AggBr. 

The object of tlm developer is to strengthen this latent 
image so as to render it visible, and to convert it into metallic 
silver. 

Several developers are used for this purpose, and we will 
consider their chemical action in turn. 

Allxiiiine Development with Pyrogallic Acid. — As a stand- 


221 


THE CHEMISTRY OF DEVELOPMENT, EIC. 

ard developer for our ordinary plates or tilms we may take the 
following formula : 


Pyrogallic acid 2 grains 

Ammonia (.880) , 2 minims 

Potassium bromide 1 grain 

Boiled distilled water 1 ounce 


When such a developer is poured upon the surface of a 
gelatine plate which has been exposed within the camera the 
following chemical changes take place : 

2Ag,Br + + 2 NH 4 HO = 2Ag, + 

Silver and Pyrogallol and Ammonia produce Silver and 
Sub-bromide 

2NH4Br + [C6H3(H0)3 + O] + H^O 

Ammonium and Oxidized Pyro and Water. 

Bromide 

The ammonia probably forms a combination with the pyro- 
gallic acid (or pyrogallol, as it is more projDerly termed) which 
may be designated ammonium pyrogallate. This substance 
attacks the silver sub-bromide but not the silver bromide. The 
result is that the bromine in the silver sub-bromide is ab- 
stracted, and metallic silver is produced. This takes place, be 
it remembered, on the surface of the him only. 

But the nascent silver has a powerful chemical action upon 
the layer of silver bromide underneath the surface layer of 
sub-bromide. It combines with this bromide and reduces it to 
the state of sub-bromide : 

Ag -I- AgBr = AgoBr 

Silver and Silver Bromide produce Silver Sub-bromide. 

Use of Soluble Bromides as Restrainers . — It is usually found 
necessary to add a small quantity of either potassium bromide 
or ammonium bromide to the alkaline pyrogallic developer. 
Different makes of dry-plates differ much as to the quantity of 


Note. — Another view of the phenomena of development was sug- 
gested to me by the well-known fact that the presence of water is indis- 
pensable. If we suppose the first chemical action that takes place to be 
the decomposition of the water, HgO^Ha f-O, then the pyrogallic acid 
will be oxidized by the oxygen, while the hydrogen will combine with the 
bromine of the sub-bromide to form hydrobromic acid, H-|-Ag 2 Br= 
HBr + Agg. W. J. H. 


222 


THE CHEMISTRY OF PHOTOGRAPHY. 


bromide which they require ; but the maker’s formula usually 
gives the proper proportion. 

When the exposure has been very short, and a weak devel- 
oper is employed, it is possible to dispense with such a “ re- 
strainer ” altogether. 

Many workers who take care to use only the best brands of 
dry-plates invariably dispense with bromide for their instan- 
taneous pictures. 

The oj0&ce of the bromide — and we may say at once that we 
prefer 'potassium bromide — is to prevent the reduction of silver 
upon those parts of the plate which have not been affected by 
light ; to save the plate from being fogged,” in fact. 

IS'ow silver bromide is soluble in a solution of potassium 
bromide, a fact which shows that the two substances have 
some chemical affinity for one another. It is probable that 
the one bromide forms a loose molecular combination (= double 
salt) with the other : 

KBr + AgBr = KBrAgBr 

Potassium and Silver produce Double Bromide 

Bromide Bromide of Potassium 

and Silver. 

The dbuble bromide is better able to resist the action of the 
developing solution than the silver bromide alone ; and thus 
the unexposed parts of the plate are kept clear from fog. 

Ferrous Oxalate as a Developer . — The use of ferrous oxalate 
as a developer was discovered almost simultaneously by Mr. 
Carey Lea in America, and by Mr. W. Willis, Jr., in the year 
1877. “ It is generally prepared by making saturated solutions 
of potassium oxalate and of ferrous sulphate, and then pouring 
(not more than) one part of the latter into three parts of the 
former. Chemical action at once takes place, and the color of 
the mixture should be a clear ruby. 

Before mixing the solutions it is well to add a few drops of 
sulphuric acid (3 or 4 to each ounce of the liquid) to the fer- 

FeSO^ "b 2K2CJJO4 K3Fe(C204)2 + K2SO4 

Ferrous and Potassium produce Potassio-Ferrous and Potassium 

Sulphate Oxalate Oxalate Sulphate. 


British Journal 0 / Photography for 1877, p. 293. 


THE CHEMISTRY OF DEVELOPMENT, ETC. 223 

rolls sulphate solution. About the same quantity of a 10 per 
cent, solution of potassium bromide should be added to the 
mixed developer to act as a restrainer. 

The above proportions (1 to 3) are the strongest permissible ; 
but it is better to use 1 to 4 ; and for lantern slides and bro- 
mide paper (for which ferrous oxalate is an admirable devel- 
oper) it should be used weaker still, say one 1 to 6. 

When a solution of ferrous oxalate is poured upon an exposed 
dry-plate, the following reaction takes place : 

6 FeCjj 04 + SKgCgO^ + GAggBr = GAgg + GKBr + 3 Fe 3 (C 204)3 
Ferrous and Potassium and Silver prod- Sil- and Potas- and Ferric 
Oxalate Oxalate Sub-bro- mcc ver slum Bro- Oxalate, 

mide mide 

Ferrous oxalate is a developer which gives particularly clear 
and brilliant negatives, and if the exposure has been correct, or 
very nearly so, it is all that can be desired ; it has not, however, 
nearly the ‘^latitude ” of pyro. 

Under-exposure may be met — to some extent — by adding to 
cacti ounce of the developer from five to ten drops of a 5 per 
cent, solution of “hypo”; this has a marked effect in bringing 
out detail.'^ 

The chemical effect of the “hypo” is to remove from the 
developing solution, or rather to convert into comparatively 
harmless substances, the ferric oxalate and the potassium bro- 
mide formed during development, both of which are powerful 


restrainers, 
follows : 

The ferric oxalate is 

acted upon by the hypo as 


+ 2 NaoS 203 = 

Na2S^O,; + 

Na2C204 

Ferric 

and Sodium produce 

Sodium and 

Sodium 

Oxalate 

Thiosulphate 

Tetrathionate 

+ 

and 

Oxalate 

2 FeC 204 

Ferrous 

Oxalate, 


The potassium bromide is also converted into sodium bro- 
mide, whose restraining action is less energetic. 

Na 2 S 203 + 2KBr = + 2NaBr 

Sodium and Potassium produce Potassium ajid Sodium 
Thiosulphate Bromide Thiosulphate Bromide. 


* Abney, in Photographic Jotirnal for 1880 ; pp. 22, 160. 


CHAPTER XXI r. 


CHEMISTRY OF ALKALINE DEVELOPMENT 
(CONCLUDED). 

Alhaline Development Acts Internally. — We have seen that 
in the ^^acid” development of wet-collodion plates, etc., the 
silver iodide in the film merely served as a foundation on 
which to form the latent image. That image was subsequently 
strengthened, built-up, or ‘^developed ” by depositing silver upon 
it from a developing solution (containing nitrate of silver, plus 
a reducer, plus an acid) which was poured upon its surface. The 
deposit of silver forming the picture thus grows upwards from 
the surface of the plate, and is composed of matter which the 
sensitive surface did not originally contain. 

In alkaline development exactly the opposite takes place. 
The image grows downwards^ and is fed and added to by 
silver already contained in the film. 

Our gelatine dry-plates and films are coated with gelatine 
containing bromide of silver (_^ gelatino-bromide of silver 
emulsion). Light forms a “latent image” on the surface, 
which image consists — for chemical purposes we may say — 
practically of silver sub-bromide, AggBr. 

The alkaline developer with which the plate is bathed sepa- 
rates these two elements, combining with the bromine and lib- 
erating the silver : 

AggBr = Ag^ + Br 

Silver sub-bromide produces Silver a?id Bromine. 

Xow when an element is set free — as the silver is in this case 
— atom by atom, it is, chemically, in a peculiarly active condi- 
tion (known to chemists as the “nascent” state). 

This nascent silver immediately attacks the molecules of sil- 
ver bromide which form a layer underneath tlie layer of silver 
sub-bromide which comj)oses tlie latent image. It combines 


THE CHEMISTET OF DEVELOPME2vTT, ETC. 225 

with this silver bromide, reducing it to the state of sub-bro- 
mide : 

AgBr + Ag = Ag^Br 

Silver and Nascent produce Silver 

Bromide Silver Sub-bromide, 

The layer of sub-bromide so formed is in its turn attacked by 
the developer, and nascent silver is again liberated. And so 
the action goes on until it passes downwards right through 
the thickness of the film and reaches the glass or celluloid at 
the back. The developed image can then be seen by looking 
at the hach of the plate ; and it consists of dark-colored reduced 
silver. 

The coating of gelatine emulsion may be put on the plate so 
heavily that it is quite a quarter of an inch thick when swollen 
by soaking, and it may be so highly charged with silver bro- 
mide as to be quite opaque, hfotwithstanding this, it will be 
found easy to develop an image right through to the glass 
support or backing. In this case the silver molecules at 
the back could not have been affected by lights and their 
conversion to metallic silver can only be explained by the 
downward growth of the image, due to the chemical action of 
the developer. 

From the same cause the image also spreads lateixdly or 
sideways. Microscopic examination of a film proves this 
clearly ; and in photomicrography it is sufiicient to interfere 
with the absolute sharpness which is desirable. 

Ahneif s Experiment . — A remarkable experiment, due to 
Captain Abney, is to expose a gelatine dry-plate in a camera 
(so producing a latent image), and then to coat one-half of it 
with collodio-1 )roniide emulsion (bromide of silver emulsified 
in collodion). The plate is then developed as usual, when it 
is found that the image on the coated half becomes much more 
dense than that on the uncoated j^art. If the film of collodio- 
bromide be then stripped away from the gelatine beneath, it 
will be found that there is an image on each. The image has 
grown upwards through the collodio-bromide (which was not 
exposed to light at all), as well as downwards through the 
gelatine emulsion. Starting from the surface of the gelatine 


226 


THE CHEMISTRY OF PHOTOGRAPHY. 


Him, the image lias been fed with silver both from above and 
from below. ^ 

This strongly reminds us of the electrolytic deposition of 
metals, as in electro-plating ; and the attraction by which each 
atom of deposited silver draws to itself other atoms of the 
same metal is beyond question of a “ polar” nature, and almost 
certainly electrical. 

delation of Develojpment to Hapidity . — The rapidity of our 
modern gelatine dry-plates is not altogether due to the superior 
sensitiveness to light of tl^e emulsion of gelatino-bromide of 
silver with which the plates are coated ; it is also in no small 
part owing to the fact that we are able with such plates to use 
a much more jiowerful developer. In the old wet-collodion, 
etc., processes the plate was covered during development with 
a solution of silver nitrate. How if an alkaline developer be 
applied to a plate upon or in the film of which there is free 
silver nitrate, a deposit of metallic silver is produced all over 
the plate, which is then said to be fogged.” In such proc- 
esses an acid developer was necessarily employed ; but acid 
developers are not nearly so powerful as alkaline developers. 
Again, in our modern dry -plates the particles of silver bromide 
are individually embedded in gelatine, and this gelatine acts as 
a physical restrainer. A developer whicli is so strong as to be 
able to reduce silver bromide when applied to that substance 
separately, cannot affect it when the silver salt is emulsified in 
gelatine. The gelatine wraps I’ouud and encloses each tiny 
particle of the silver bromide, and causes the chemical action 
of any developer to be slow and steady. This gives time for 
other forces, as electricity, to play their part. 

Owing to the numerous developers now employed, and to the 
fact of a somewhat general similarity of appearance between 
them (especially when made up as solutions), it is often useful 
to know how to distinguish them from one another. The fol- 
lowing table* affords the means of doing this : 


=5= By L. Van Neck, in the Bzilletiji Beige, 1890. 


REACTIONS 


OF VARIOUS DEVEf^OPERS. 


227 


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CHAPTEK XXIII. 


ORTHOCHROMATIC PHOTOGRAPHY. 

Difference Between the Optical and the Chemical 
Effects of Light. 

What is light ? It is not a thing it is not any kind of 
matter, for it has no weight. And yet light and matter are 
inseparable. Light is one of the physical forces — it is grouped 
with heat, sound, gravity, electricity, etc. — and is consequently 
a state or affection of matter. 

Two' centuries back the mighty Xewton argued that light 
was matter : maintaining in his “ Corpuscular Theory ’’ that all 
light-giving bodies continually emitted infinite numbers of 
tiny particles or corpuscles,” which travelled through space ; 
and that our sense of sight was due to the striking of some of 
these corpuscles upon the retina or sensitive expansion of the 
optic nerve which lines the back of the eye-ball. 

But Newton’s corpuscular or emission theory of light — 
though long upheld by his great name — has been disproved 
by the researches of Huyghens, Young, etc., who discovered 
that light is due to the rapid motion of the molecules of self- 
luminous bodies ; this motion producing waves in a medium 
called the ether, which is believed to fill all space. The ether 
is a gaseous substance of almost infinite tenuity — we should 
have to weigh it by the cubic mile to find that it had any 
weight at all — and it fills the universe, occupying all the space 
between the earth and the sun, moon and stars. It is able 
to pass through even solid bodies with the greatest ease ; in- 
deed, Sir W. R. Grove remarks that the ether passes through 
the earth as easily as the wind through a forest of trees.” 

The waves produced in this ether by luminous bodies pass 
through it with the almost inconceivable velocity of 186,000 


ORTHOCHROMATIC PHOTOGRAPHY. 


229 


miles per second ; and, when they strike upon the retina of 
the eye, they set the fibres of the optic nerve in motion, and 
this produces in the brain the sensation of light. This wave 
motion can be imitated by throwing a stone into the middle 
of a pond. Waves are produced which spread in circles until 
they reach the margin. The light-weaves from the sun take 
rather more than eight minutes to reach the earth ; but the 
nearest fixed star is at such an enormous distance that an un- 
dulation or wave of light leaving that star, for example, on 
the 1st of January, 1893, would not reach us until the end of 
June, 1896. 

Let us conceive of light, then, as a kind of motion ; and 
compare the waves of ether falling upon the substance of our 
earth, with the waves of w^ater which we know are able to 
produce such tremendous effects upon our coasts. The waves 
of light do their w^ork more quietly than the weaves of w^ater, 
but their action is none the less real, and of far greater im- 
portance. If our eyes were able to distinguish these w-aves 
of light, and also the molecules of silver salts with which our 
dry-plates are coated, we should see the ether dashing upon 
the molecules and breaking them to pieces, just as we may see 
the mighty w^ater waves of the Atlantic battering beach and 
cliff in a spring storm. 

Mathematically and by experiment it has been demonstrated 
that the undulatory or w^ave theory of light is the true one. 

In the study of light (which is called optics) w^e call the 
direction of the line along which the light-wave passes, a ray 
of light. 

Compound Haiuee of White Light. 

When a ray of sunlight is caused to pass through a three- 
cornered ]iiece of glass known as a prism (of which the 
‘Alrops” of lustres, some chandeliers, etc., are examples) it is 
found to be spread out (on the other side where it emerges 
from the prism) into a band of colored light. This colored 
band really consists of an infinite variety of colors, but it is 
usual to distinguish seven principal colors — red, orange, yellow, 
green, blue, indigo and violet. The word ^‘roygbiv” wdll 


230 


THE CHEMISTRY OF PHOTOGRAPHY. 


prove an aide memoire for these colors, as it gives the first letter 
of the name of each color in its proper order of succession. 
Thus the prism shows white light to be a compound of all the 
colors of the rainbow, the latter natural phenomenon being 
indeed produced by the passage and consequent decomposition 
of (white) sunlight through minute prisms of ice or drops of 
rain high up in the air. 

The cause of the decomposition of a ray of white light into 
a colored band by passage through a prism is that the light is, 
in the first place, bent or refracted by passing from the air to 
the glass on entering the prism, and still more bent (in the 
same direction) where it emerges from the prism. But the 
second and main reason is that the variously colored parts of 
the ray are not refracted equally, the red rays being bent the 
least and the violet rays the most. The former are known as 
the Jess and the latter as the more refrangible rays; and the 
entire band of colored light is called the spectrum. The spec- 
trum of sunlight is crossed by a number of vertical black 
lines (called Fraunhofer lines, after the German optician who 
first mapped them in 1814). These lines are due to the pres- 
ence of certain elements in a vaporous state in the atmosphere 
of the sun, and they are not seen in the spectrum of the elec- 
tric light or of the lime-light, etc. 

The Fraunhofer lines serve usefully as an index to the vari- 
ous parts of the spectrum, as they never change their posi- 
tion. The principal lines are indicated by the letters of the 
alphabet ; A is at the extreme end and H in the middle of the 
red ; C separates the red from the orange ; D is in the yellow, 
E in the green, F in the blue, G in the indigo, and II m the 
violet. 

An instrument specially constructed to study the spectra of 
bodies is called a spectroscope. It consists of a narrow slit 
through which liglit is admitted ; a lens or lenses to render the 
rays parallel ; a prism or prisms to decompose the white light ; 
and a small telescope through which the spectrum so produced 
can be examined. 

Every one who has watched water-waves — either from the 
deck of a vessel or from a beach — must have noticed that 


ORTHOCHEOMATIC PHOTOGRAPHY. 


231 


sometimes the highest parts or crests of the waves are close 
together, while sometimes they are far apart. The length of 
a wave is measured in a straight line from crest to crest. 
Sometimes this is only a foot or two, but it may be some hun- 
dreds of times greater. The wave-lengths of light are almost 
inconceivably short, but they have been accurately measured, 
and the results as expressed in tenth-metrets ” or ten-mil- 
lionths of a millimetre* are 


Beyond the red there are rays — the “ infra-red ra.ys — 
which are invisible to our eyes, but which have much heating 
power, strongly affecting the thermometer. These have been 
mapped as far as wave-length 10,000 by Abney, and they have 
been detected down to wave-length 22,000. Beyond the violet 
there are other invisible rays — the ‘‘ultra-violet” — which have 
a powerful chemical action ; these have been traced as far as 
wave length 2,000 by Hartley. 

Centres of Heat, of Light, and of Chemical Action in 

the Spectrum. 

Every part of the spectrum is capable of producing heat, 
light, and chemical action. As to the two former, no doubt 
the point of greatest heat lies in the “ infra-red,” and is there- 
fore produced by rays which are incapable of affecting the 
retina. The point of maximum luminosity or brilliancy is in 
the yellow-green. But the chemical action of the differently- 
colored rays depends largely on the substance which is chosen 
to receive them. For the sensitive salt — ordinary silver bro- 
mide — which is generally used on our dry- plates, the blue rays 
are the most powerful. 

*The millimetre = .03937 inches, or the twenty-fifth part of an inch nearly. 


A = 7G04 
B = 6867 
C = 6562 
D = 5892 


E = 5269 
F = 4861 
G = 4307 
H = 3968 


232 


THE CHEMISTRY OF PHOTOGRAPHY. 


The three curves shown in figure 1 represent, by the height 
of each curve above the base line, the powers of heat, of light, 
and of the chemical force possessed by each part of the 
spectrum. 

With the heat rays we are but little concerned in photog- 
raphy ; we do not secure our pictures by their aid, but by the 
aid of the rays of light. 

At first sight it may seem fortunate that those parts of the 
spectrum (the yellow, green, and red rays), which can exert 
but little chemical force, are those which are most luminous, 
producing the greatest efiEect upon our eyes ; while the blue 



Fig. 1 . — Diagram of the various intensities of light, heat, and chemical 
action throughout the spectrum. 

The height of each curve above the horizontal line indicates 
the intensity at that point. The rays which do not affect the eye 
are shaded. The Fraunhofer lines are lettered from A to H. 


and violet rays, which really do the work of “taking pictures” 
on our dry-plates, are comparatively dark as seen by the eye. 
This enables us to take our photographs by the aid of the blue 
and violet rays, and then to develop them in a room lighted by 
tlie yellow or red rays. 

Yet further thought will show that as a fact this state of 
things is most unfortunate. Photographs so taken cannot be 
true transcrijits of natural objects. The sensitive dry-plate 
“sees” an aspect of nature, but it is not identical with that 
whicli is impi’essed u])on tlie retina. In the resulting prints 
the yellows, greens, and reds come out far too dark; the blues 
and violets too light. 


OKTHOCHROMATIC PHOTOGRAPHY. 


233 


The illuminating and chemical forces respectively of the 
variously colored rays are shown in the following table : 


Colors. 

L.uminous 

Intensities. 

Chemical Intensities on 
Silver Bromide. 

Lines. 

Dark Red 

just perceptible. 

0 

A 

Red . - 

33 

5 

B 

Bright Red 

94 

10 

C 

Orange 

640 

50 

D 

Yellow 

1000 

100 


Green 

480 

200 

E 

Blue 

170 

500 

F 

Indigo 

31 

1000 

G 

Violet 

6 

650 

H 

Ultra Violet 

0 

450 



Thus a light yellow, just bordering on the green, is the 
brightest or most luminous color ; next to this come orange 
and green ; while dark blue (indigo) and violet are the darkest 
colors. 

But the chemical effect is shown by the table to be very 
different ; the maximum effect is here produced by the blue 
end of the spectrum. 

In figure 2 the chemical effects of light upon the three sen- 
sitive salts of silver — the chloride, the iodide, and the bromide 
— are compared, graphically, with the luminous effects of the 
differently colored rays. It will be seen that each of the three 
curves representing chemical effects diher widely from the 
curve of luminosity. 

The question then arises — Is it possible to obtain a substance 
which shall ‘‘see” chemically as w^e see ; which shall be most 
sensitive (like our eyes) to yellow rays, and least so to dark 
blue ; with the other colors producing chemical effects in pro- 
portion to their luminosity. Our photographic plates w^ould 
then furnish an accurate transcript — in black and white — of 
nature ; their “ tones ” would be correct, as the artist says. 
The attempts to bring the chemical curves into coincidence 
with the curve of luminosity have resulted in the discovery 
of orthochromatic photography ; and of this we shall next 
proceed to treat. 


234 


THE CHEMISTRY OF PHOTOGRAPHY. 


In the preceding paragraphs we liave tried to explain how 
differently the many colors of which wliite light is composed — 
and which we may here for convenience sake reduce to two 



Fig. 2. — /., silver chloride; //., silver iodide; III., silver bromide ; 

luminosity of the spectrum ; F., Fraunhofer lines in the solar 
spectrum. 

The height of the curve above the horizontal line in each case 
shows the intensity of the action at that point. 


only, yellow and blue — affect our eyes and our dry-plates 
respectively. To our eyes the yellow looks bright and the 
blue dark, while we all know that if we photograph on an 
ordinary plate an Italian peasant, dressed in a blue skirt and 
a yellow bodice, the resulting print will show tlie skirt white, 
or nearly so, and the bodice black. 

The keen eye of Mr. Punch ” noted this long ago, and in 
the issue fora certain week in July, 1861 , we have a droll 
picture of a footman showing his photograph to the house- 
maid. “ ^Vhy, Tummas,” says the delighted Mary, “ it’s the 
very moral of yer ! ” Pretty thing, ain’t it,” replies 
Thomas. ‘‘ Pity the yaller of the uniform comes so black ! ” 

Several eminent professional photographers have written 
pamphlets of instructions for their lady sitters, pointing out 
how the color scale is changed by photography. But sitters 
with auburn (to put it mildly) or golden hair, have always 
been a troulile to the operator, who usually resorts to powder- 


OETHOCHROMATIC PHOTOGRAPHY. 


235 


ing sucli hair plentifully as a partial remedjj while the freckles 
which as often as not enhance the beauty of a blonde, are re- 
produced as transparent spots on the negative (black on the 
print), giving no end of trouble to the retouclier. 

Then as to copying pictures. What a parody is a copy, on 
an ordinary plate, of an old master,” or a harvest scene, or a 
sunset, or indeed any picture which depends mainly on the 
artist’s skilful use of color for its effects. 

All these defects were noted in the early days of photog- 
raphy — thirty or forty years ago — and sundry attempts were 
made to remedj^, or at all events mitigate, the faults com- 
plained of. 

An editorial “ article in the Photograpliic News for Oct. 
15, 1858, On Copying Paintings by Means of Photography,” 
suggests the use of the bromide of silver instead of the iodide 
(the latter being the salt of silver in general use at that time) 
and goes on to recommend the placing of a solution of sul- 
phate of quinine in a glass cell in front of the lens. The 
quinine absorbs and quenches most of the violet and ultra- 
violet raj^s ; whose action on the silver salt is so great, on the 
eye so small. Crookes adds : A thin piece of yellow glass 

employed in the same way will act even more vigorously, and, 
were it not for the uneven surface which this kind of glass 
usually has, it would answer the purpose admirably.” Experi- 
ments showing the effect of using as many five thicknesses 
of yellow glass in front of the lens when photographing bril- 
liant colors are then detailed : — “ At this stage the photographic 
effect of the different colors was much nearer their true effect 
on the retina than if they had been copied in the ordinary 
way, but they were still very far from giving the tones which 
an engraving of the same subject would have presented. The 
too energetic action of the blue color was entirely overcome, 
but the red and yellow still offered difficulties which, we fear, 
no amount of obstruction would ever have properly overcome.” 

In this article the true function of the yellow screen is cor- 
rectly pointed out. Its use is to subdue the blue and violet 

* William Crookes, the famous chemist, then edited the News, and the article is presum- 
ably by him. 


236 


THE CHEMISTRY OF PHOTOGRAPHY. 


rays, reducing tlieir too energetic action to a more manageable 
one. Blit on ordinary films the action of the remaining part 
of the spectrum — the greens, yellows, and reds — is so slight 
that the time of exposure is so much increased as to make the 
method, by itself, valueless. Moreover, even if sufficient time 
be allowed, the general effect is flat, and even foggy. 

As far as the unevmn ” character of the yellow glass is 
concerned, that is now obviated. For Mr. J. It. Gotz, of 19 
Buckingham Street, Strand, London, has supplied us with 
sheets of yellow plate-glass, the twenty-fifth part of an inch 
in thickness, which can be had in four different depths of tint, 
and which answers admirably when used in conjunction with 
orthoohromatic plates. Similar screens can now be had from 
several other firms. 

Having found that it is useless to hope to obtain a correct 
tone, or tonality,” by excluding those rays (the most refran- 
gible — blue, violet, and ultra-violet) which are most active or 
“ actinic” in bringing about chemical changes, it remains to 
be seen whether it is not possible to exalt or increase the sen- 
sitiveness of the silver salts which we employ for the less re- 
frangible rays, the yellow, green, and red. 

Draper, Hunt, and many others of the early wmrkers in 
photography, imagined that there were three distinct spectra 
laid one upon the other, as it were. The chemical or actinic 
spectrum they thought extended from the red to the ultra- 
violet rays, but it was nearly or altogether neutralized at the 
red end by the other overlapping spectra. At its free end 
(the ultra violet) its rays were capable of producing the most 
powerful actinic effects. Similarly the heat spectrum had its 
free end beyond the red ; for the ultra-red rays are, we know, 
those which possess most heat. Lastly, tlie luminous sjDectrum 
occupied the centre (overlapping the others on each side) and 
its greatest power of luminosity lay in the yellow. 

But later researches have shown that this theory is alto- 
gether incorrect, and that in examining into the powers and 
qualities of the various rays which constitute what we call 
the ^‘spectrum” of white light, we must consider the sub- 
stance u])on which the raysfall^ as well as the rays themselves. 


ORTHOCHEOMATIC PHOTOGRAPHY. 


237 


We call the blue and violet, etc., rajs actinic’’ and 
chemical ” simply because the substances we usually employ 
in photography — the salts of silver — are in their normal state 
most readily affected by these rays. But there are other ma- 
terials upon which the yellow and red rays are most effective, 
chemically speaking. So, also, it is a fact that the yellow 
rays affect our eyes most powerfully ; but there are good 
reasons for believing that to some of the lower organisms it 
is the violet light which is most luminous. 

The founder of orthochromatic photography is Professor 
Hermann W. Y ogel, of Berlin. In an article which appeared in 
the Photogrcqyhic News for December 12, 1873, he announced 
that if bromide of silver be dyed by means of certain yellow 
or red dyes, it becomes sensitive to yellow or red light. Yogel 
used principally dry-plates coated with an emulsion of silver 
bromide in collodion. The first dye he used was the sub- 
stance known as corallin,” which he dissolved in alcohol and 
added to the collodion emulsion until its color was a ‘^vigorous 
red.” Plates coated with this colored emulsion were “nearly 
as sensitive to yellow light as to indigo.” Aniline-green was 
also tried, which was found to sensitize for the red rays. A 
crucial experiment is described in this paper. “ A blue band 
upon a 3 ^ellow ground w^as photographed. With an ordinary 
iodide of silver collodion plate, I obtained a white band upon 
a black ground. AVith a bromide of silver coralline plate, 
upon which blue and yellow acted with equal power, nothing 
could be obtained ; I foresaw, and for this reason I put in front 
of the lens a yellow glass plate, which absorbed the blue light, 
and allowed the j^ellow rays to pass through unheeded; and 
then I was enabled to obtain, after a sufficiently long exposure, 
a dark band upon a light ground.” 

Numerous contributions on the subject from Professor 
Yogel’s pen appeared in the Pliotogro.j)hiG N^eics during 1874- 
5-6 and 7. His conclusions were opposed, or doubted, bv 
Monckhoven, Carey Lea, Spiller, and others, but his arguments 
were sound, his work was true, and they received valuable 
confirmation from Capt. AVaterhouse, of Calcutta, who in 1875 
photographed the green, yellow, and red rays of the spectrum 


238 


THE CHEMISTEY OF PHOTOGEAPHY. 


(as well as the more refrangible blue and violet) by using 
plates stained with the pink dye called roseine, with Jud~ 
son’s” dyes (including turmeric), with chlorophyll, etc. 

In 1876 Waterhouse for the first time added a red dye 
known as eosin to a collodio-bromide emulsion, and with good 
results. Yogel’s experiments were conducted mainly on the 
spectrum, but Waterhouse tested his eosin-collodion plates on 
landscapes. He writes: “From this remarkable sensibility to 
the green and yellow rays, it might have been anticipated that 
wet plates prepared with the eosin-stained collodion would 
show an increased sensitiveness for foliage and other colored 
objects of a green or yellow tint. On trying a landscape, I 
found that though the collodion was by no means strongly 
stained, the exposure was increased to about three times what 
was necessary for wet collodion. There was no marked 
increase of detail in the foliage, but, if anything, a decrease. 
There was, however, a great increase in the density of the 
image and in the clearness of the shadows. Subsequent trials, 
both with dry bromide aad wet bromo-iodide plates, on 
bouquets of fiowers and a stained-glass window, comprising 
red, green, blue, and yellow, showed that but little practical 
advantage was to be gained by the use of the stained collo- 
dion, though the plates were undoubtedly more sensitive to 
yellow than is ordinarily the case, and showed the same 
increase of intensity, which may be a further advantage. 

“ I have not yet had leisure to fully investigate the action 
of the dye ; but this brief record of a few results obtained 
with it may be of interest as bringing to notice a coloring 
matter which in some degree supports Dr. Yogel’s theory, that 
a film of dry bromide of silver may be rendered sensitive to 
certain rays of the spectrum by being stained with a color 
which absorbs those rays — and also as showing that the photo- 
graphic action of the spectrum is but a slight index to the 
action of the colored objects around us.” 

We have quoted these paragraphs from Capt. Waterhouse’s 
paper because he was the first to use the dye — eosin — which is 
the one now mainly used for orthochromatic photography; 
because he was the first apparently to test his color sensitized 


ORTHOCHROMATIC PHOTOGRAPHY. 


239 


plates on landscapes ; and because lie so correctly points out 
that photographing the spectrum with its band of pure colors 
is one thing, while photographing the colored objects found in 
nature is quite another. Scarcely any colored object reflects 
or is visible by pure monochromatic light. The brilliant red 
of many flowers will be found (when examined sjDectro- 
scopically) to contain a proportion of blue rays, etc. But 
more than this, all objects reflect — in addition to any proper 
color which they may possess — a certain amount of white lights 
and it is by this reflected white light that they are photographed. 

Thus we do not photograph the green leaves, or grass, by 
their green light, but by the white light which they reflect in 
addition to the green. This reflection varies with the angle 
of each leaf, and this explains the common spottiness of 
foliage, one leaf — at the proper angle for reflecting white light 
into the lens — coming out quite white in the print, while its 
neighbor — at a different angle — is quite black. 

One of the first scientists to confirm and support the discov- 
eries of Professor Yogel was the famous French savant^ 
Edward Eecquerel. In the latter part of 1871 he communi- 
cated to the Paris Academy of Science the results of experi- 
ments on films stained with coralline and with aniline green, 
which showed an action in the yellow and green similar to that 
obtained by Yogel. But Becquerel went further: he used 
chlorojphyll^ the green coloring matter of plants, to stain a 
collodio-bromide emulsion. Plates coated with this emulsion 
and exposed to the spectrum showed an action not only in the 
blue and violet, but also in the green, the orange, and the red. 
Becquerel also noticed that the points of maximum action on 
the plate corresponded with the absorption lines of the chloro- 
phyll. As to these absorption lines, we may explain that when 
solutions of different bodies are examined by means of trans- 
mitted light with a spectroscope, we commonly see dark bauds 
— ‘‘absorption bands or lines” — in their spectra. These dark 
spaces show that the light has been there absorbed by the 
substance. 

Now light is a form of energy, and we know that when one 
kind of energy disappears, another kind appears — for “ energy 


240 


THE CHEMISTRY OF PHOTOGRAPHY, 


is indestructible.” What becomes of the lost light? It is 
probably converted into either heat or chemical action, or 
more probably into both of these forms of energy. 

The first writer to make any practical nse of Vogel’s dis- 
covery was Ducos Duhauron, who employed* collodio-bromide 
containing coralline, etc., to obtain the negatives he desired to 
employ in his process of Heliochromy,” or printing in colors. 
We do not refer, except incidentally, to heliochromy in these 
articles, but in the appendix references to it are included. 
Orthochromatic photography is indispensable to heliochromy. 

Duhauron in 1878 went further, and employed improved 
methods of a similar description. 

After 1877 orthochromatic photography went to sleep for a 
few years. Vogel — its discoverer — had only a scientific in- 
terest in it ; and the work of Waterhouse seemed to show that 
it had not much practical utility. 

Then in 1879-80 came the great revolution — the displace- 
ment of collodion by gelatine. Seeing that gelatine dry- 
plates were so greatly superior to collodion in rapidity, it was 
natural to study their other comparative properties also. One 
might have imagined that the effect of dyes upon such gela- 
tine plates — the attempt to sensitize them for the less refran- 
gible rays — would soon have been made, but it was not so. 

In February, 1882, Captain Abney used bromide paper dyed 
with eosine in his Cantor Lectures delivered before the Society 
of Arts. 

After an interval of six years the first move came from 
France. On 8th January, 1883, a patent (specification Ho. 
101) was taken out in England by C. D. Abel, on behalf of 
Pierre Alphonse Attout, called Tailfer, and John Clayton ” for 
the application of eosine to gelatino-bromide emulsions or 
coated plates to get the relative lights and darks correctly in 
spite of varying color. 

The eosine is dissolved in ammonia,f and added to the 
emulsion at the moment of its formation. 


* Photographic News, June 5th, 1874. 

t The other or “ fixed ” alkalies— as the carbonates of soda or potash— are of no use in 
preparing isochromatic plates. When the plates are dried they crystallize. 


ORTHOCHROMATIC PHOTOGRAPHY. 


24:1 


“ Piates that are already coated and dry have the ammoniacal 
solution of eosine to which alcoliol has been added, poured 
over the film. They are washed, the eosine remaining fixed. 

The term eosine is used inclusively : and other alkalies 
than ammonia may be used.” 

The same principle had been patented a few weeks earlier 
in France. The date given above is merely that on which the 
patent was ‘‘ handed in ” ; the specification would not he^^uh- 
lished till at least six months later. 

At first the “ Attout-Tailfer ” found but little favor 

in England ; and it was not until a medal was awarded to 
them at the Inventions Exhibition, held at South Kensing- 
ton, London, in 1885, that their merits began to be recognized. 
The English patent was soon afterwards purchased by Messrs. 
B. J. Edwards & Co., The Grove, Hackney, London, who 
advertised them as ^Hsochromatic” ^ plates. They were 
brought still further into notice by the work of Messrs. Dixon 
& Gray, who exhibited some wonderfully good copies of oil 
paintings at the Pall Mall Exhibition of the Photogra])hic 
Society of Great Britain, held in 1886 ; coj)ies which had been 
made upon plates prepared in a similar way to that described 
in TailfePs patent. Ever since that date the use of isochro- 
matic plates has increased in Great Britain, and for copying 
paintings, flowers, etc., and for the highest class of studio 
work they are now considered indispensable. 

It will be noted that the Tailfer patent claims the use of 
‘‘eosine,” and this term includes a very large number of dyes, 
of which the variety known as “ erythrosine” is that now used 
in the manufacture of the isochromatic plates; it also includes 
the use of any alkali with the dye, and this is another impor- 
tant point. 

At about the same period (1882-6) that the Tailfer plates 
were being introduced into England, rumors came from time 
to time from Germany that the famous house of A. Braun et 

* From the Greek zV^jj-equal, and c/iroma-co\or ; not a very happy word, since our ob- 
ject is not to render all colors equal in their effect upon our dry-plates, but to cause them 
to produce the same chemical effects as they do luminotis effects. The word “ isochro- 
matic ” was suggested for the Tailfer plates in 1882 by the eminent French statesman, M. 
Paul Bert, at that time professor at the Sorbonne in Paris. 


242 


THE CHEMISTKY OF PHOTOGRAPHY. 


Cie, of Dornach and Paris, Lad discovered some “ secret col- 
lodion process ” by which they were able to obtain photo- 
graphic copies of oil paintings, etc, superior to anything that 
had been done before. The secret process” was, no doubt, 
the use of collodio-bromide dyed plates, but the firm naturally 
did not publish their method. In 1884 Messrs. Braun were 
permitted to erect a temporary studio in front of the National 
Gallery, London, and to copy between 300 and 400 pictures 
belonging to the famous collection there exhibited. The 
prints showed a marvellous improvement in the rendering of 
the reds and the yellows, and it was argued by many that 
such an improvement must be due to some system of elabo- 
rate retouching. But an eminent English worker was per- 
mitted to visit Dornach and examine the negatives, and he 
testified that the results were not due to retouching. Of course 
we now know w^ell enough that by the combined use of 

eosine ” and a yellow screen similar results are easy of at- 
tainment, but the general incredulity on this point which pre- 
vailed so recently as 1886, when Mr. Bird read his paper be- 
fore the Photographic Society of Great Britain, shows the 
rapid advance which has since been made. 

After the general recognition of the immense value of color- 
sensitized plates (at all events for special purposes) in 1885-6, 
Dr. Yogel probably saw what a mistake he had made in not 
at once adapting his process — originally introduced for collo- 
dion in 1873 — to gelatine dry-plates. For one thing, Tailfer 
had not attempted to take out a patent in Germany, so the 
matter had perhaps not been borne in upon the learned 
Doctor’s inner consciousness as it otherwise might have been. 

Tlie English patent of “Dr. Hermann Wilhelm Yogel, of 
124 Kurfursten Strasse, Berlin, Chemist,” is dated 29th No- 
vember, 1886, and is for “ An improved process for manufac- 
turing isochromatic emulsion plates highly sensitive to light.” 
The preamble states that “ In the year 1873 I made the dis- 
covery tliat chloride and bromide of silver, which are only 
sensitive to violet, indigo, and the blue light of the spectrum, 
can be made sensitive to green, yellow and red rays by mix- 
ing the above-mentioned salts with bodies that absorb the 


ORTHOCHEOMATIC PHOTOGRAPHY. 


24:3 


latter rays. If, for instance, chloride or bromide of silver is 
mixed with aniline red, which absorbs yellow rays, the chlo- 
ride or bromide will become sensitive to yellow rays ; or if 
mixed with aniline green, which absorbs red rays, the same 
will be made sensitive to red rays. I denominated these 
bodies, which sensitize silver salts in the aforesaid manner, 
ojjtioal sensitizers^ and I and others after me have discovered 
a large number of such optical sensitizers amongst the dyes, 
and in this manner the so-called isochromatic gelatine plates, 
colored with an optical sensitizer such as cyanine, chinoline 
red, eosine, and erythrosine, are now articles of commerce. 

“All these ‘isochromatic’ gelatine plates are generally less 
sensitive than ordinary plates, and require a yellow glass plate 
interposed between the lens and the sensitive plate, for di- 
minishing the power of the blue light. By this interposition 
of a yellow plate the time of exposure is lengthened, and if 
the surface of the plate is not quite even, the sharpness of the 
photograph is lost, so that these ‘ isochromatic ’ plates are used 
on a small scale only for the reproduction of pictures or paint- 
ings, but not for portraits or landscape photograj3hy.” 

“Now 1 have succeeded in making isochromatic gelatine 
plates, the sensitiveness of which is twice as great as that of 
the ordinary gelatine plates, and which do not require any 
yellow plate or screen. This discovery is based : 

1. On the application of the eosides of silver — that is, the 
chemical combinations of eosine dyes with silver. These 
eosine dyes, or derivatives of fluoresceine, are all acids, and 
combine with silver to form salts. 

2. On the addition of silver salts to other optical sensitizers, 
for increasing their sensitizing power. 

As early as 1884, I observed this favorable influence of the 
presence of silver salts, and proved, for instance, that eoside 
of silver wdll give ten times more sensitiveness for yellow light 
than ordinary eosine, but I have only now succeeded in 
making plates without fog, or film, and spots, so that I 
can introduce the process into practice, and I have j^roved that 
it is possible to produce in this manner landscapes and por- 
traits far superior to those taken with ordinary jDlates. 


2U 


THE CHEMISTRY OF PHOTOGRAPHY. 


I have now invented the following methods for making 
highly sensitive isochromatic plates or sensitizing solutions, so 
that any photographer or amateur can prepare his own isochro- 
matic plates. In order to attain this object I proceed as follows : 

1. Ordinary gelatine plates are bathed in a solution of a 
soluble salt of silver (1 to 1000), then in a solution of 
an eosine dye, or a mixture of eosine dyes with other optical 
sensitizers, with or without liquor of ammonia. The solution 
of the dye can also be used first, and the silver solution after- 
wards, or an emulsion can be mixed with a soluble salt of 
silver and the dye added to the mixture, or mce versa^ 
and with or without ammonia. 

2. A silver eoside is formed by mixing a solution of an 
eosine dye, or a mixture of different eosine dyes (for instance, 
ordinary eosine, blue-tinted eosine, chrysoline, aureosine, 
phloxine, rose Bengal, or any other derivative of fluoresceine) 
with a soluble salt of silver, such as sulphate, nitrate, acetate, 
or fluoride of silver. This eoside of silver can be collected as 
a precipitate, washed, and mixed with the emulsion, or dis- 
solved in diluted ammonia or diluted acetic acid, and employed 
as a bath for soaking dry-plates ; but I prefer to add liquid 
ammonia, carbonate of ammonia, or acetic acid, during the 
process of precipitation, so that the suspended precipitate is 
dissolved, and to employ the same as sensitizing agent for 
fluid emulsions, or the said solution can be diluted with water, 
and used as a bath for dry-plates. The quantity of this solu- 
tion of eoside of silver to be employed varies according to the 
quality and kind of emulsion treated with the same, although 
the following formula will serve as a general guide for those 
employing my sensitizing solution : 50 cubic centimetres of a 
solution of an eosine dye (1 part dye to 1000 parts water or 
diluted alcohol), 1 cubic centimetre of a solution of nitrate of 
silver (1 part nitrate of silver to 20 parts of water), 1 to 2 
cubic centimetres of liquor ammonia. This solution is either 
mixed with the emulsion — that is, 5 to 10 per cent, is added to 
the emulsion — or the same is diluted with 200 to 500 cubic 
centimetres of watei*, and the gelatine plates immersed or 
soaked in the same for about one minute, and then dried. 


OJRTHOCHEOMATIC PHOTOGRAPHY. 


245 


3. I furthermore use other dyes, known as optical sensi- 
tizers, which do not chemically combine with silver, but the 
sensitiveness of which is much improved by the presence of a 
soluble salt of silver — such, for instance, as cyanine, chinoline 
red, coeruleine, etc. — and mix them with any soluble salt of 
silver. I employ, for instance, 50 cubic centimetres chinoline 
red, or a mixture of chinoline red and cyanine (solution 1 to 
1000), wdtli 3 to 5 c.cm. of nitrate of silver (solution 1 to 20), 
and 15 c.cm. of liquor ammonia. This solution is employed 
in the same manner as the eoside of silver solution described 
under (2). 

I furthermore employ the solutions described under (2) and 
(3) in varying proportions for improving the sensitiveness for 
any part of the chromatic spectrum. 

4. I likewise employ, in like manner as mentioned under 
heads (1), (2), and (3), the salts of lead, which produce, when 
mixed with an eosine dye, eoside of lead, which said salts can 
be employed alone, or mixed with eoside of silver. 

I furthermore employ the insoluble salts of silver, such as 
the chlorides, tartrates, citrates, etc., dissolved in ammonia or 
acid. I do not confine myself to any of the preparations of 
the mixtures as given in the foregoing specification, as the 
same can be varied according to requirement, without in any 
way departing from the nature of my invention. 

Having now particularly described and ascertained the 
nature of my said inventio]i, and in what manner the same is 
to be performed, I declare that wdiat I claim is : — 

{a) The employment of the chemical combinations of silver 
or lead, with an eosine dye or dyes, for producing highly sen- 
sitive isochromatic emulsive gelatine plates. 

Qj) The employment of dyes, known as optical sensitizers, 
in combination with the salts of silver. 

(c) The employment of various dyes in combination with 
soluble salts of silver. 

{d) The manufacture of combinations or solutions of dyes 
and silver, as strong optical sensitizers for coloring gelatine 
emulsions, or as a bath for gelatine plates. 

ie) The application of silver salts not soluble in water, but 


246 


THE CHEMISTRY OF PHOTOGEAPHY. 


in ammonia or acid, for the purposes set forth in the fore- 
going specification. 

A “common sense” opinion of the patents of Attout- 
Tailfer and of Yogel, would be that they were in opposition 
to each other; but that Dr. Vogel having published to the 
world in 1873 his cardinal discovery that silver-salts could be 
“ color-sensitized ” by means of dyes — neither of them could 
be valid. But the English law is a curious thing — while Eng- 
lish patent law is, well, more curious still. In America care- 
ful inquiry into the validity of a patent is made before it is 
granted, and only a few months ago an inventor known to us 
who attempted to patent there an article of use in photography 
(he obtained his English patent without the slightest difficulty) 
received a reply from Washington directing him to consult 
the Photographic JVews for 186-, p. — , where he would find a 
full description of his “ invention.” 

At the date of writing Messrs. B. J. Edwards & Co. possess 
the sole patent for manufacturing “ Isochromatic ” plates in 
the British Isles, while Dr. Vogel’s agent (Mr. J. B. Gotz, of 
19 Buckingham Street, Strand, London,) sells Dr. Vogel’s 
“ Orthochromatic ” plates, manufactured in Germany by 
Obernetter. The Ilford Company also manufacture isochro- 
matic plates in England, under a license from Mr. Edwards. 
Dr. Vogel, in 1889, sold a solution of quinoline red and 
cyanine in alcohol, under the name of “ azaline.” When 
ordinary plates were bathed in water to which a few drops of 
this tincture had been added, they became orthochromatic. 

But the modern photographer strongly objects to unneces- 
sary trouble ! If he can buy plates already color-sensitized, 
he will not take the trouble to bathe and dry ordinary plates, 
and azaline has been but little used in this way. 

In America we believe Mr. John Carbutt has been the only 
maker to prepare commercially an “orthochromatic” dry-plate. 

* The term ‘'orthochromatic” is derived from the Greek, orthos correct; and chroma, 
color; and seems some-what preferable to “isochromatic,” although priority should be 
considered. The Rev. F. C. Lambert, M.A., has proposed the name Kairo-la7nprotic 
(from the Greek, Kairos, right proportion, and lamprotes, brilliancy) for color-sensitive 
plates; while M. Leon Vidal has still more recently suggested that “ orthoscopic,” seen 
correctly, would be a still better name. 


CHAPTER XXIY. 

THE CHEMISTRY OF SILVER PRINTING : 

MATT-SURFACE, ALBUMEN, COLLODION, AND 
GELATINE PRINTS. 

Printing With Silver Chloride on a Matt-Surface. 

The first successful ‘‘photographs” were obtained by expos- 
ing opaque or semi-opaque objects to the action of sunlight 
wliile upon and in contact with some flat surface coated with 
a sensitive salt of silver. 

As far back as 1727 J. H. Schulze printed letters and 
signs by means of sunlight upon a mixture of nitrate of silver 
and chalk. The great Swedish chemist, Scheele, made a clas- 
sical investigation into the action of light upon silver chloride 
in 1777 ; and between 1790 and 1802 Thomas Wedgwood 
and Humphry Davy obtained prints of “ paintings on glass,” 
leaves, wings of insects, and the shadows or profiles of opaque 
objects by means of paper or leather coated sometimes with 
nitrate of silver, sometimes with chloride of silver. But as 
even such a skilful chemist as Davy was unable to devise any 
means of “ fixing” these prints, the process was abandoned as 
useless. 

Talbofs “ Photogenic PrawingP — Between 1835 and 1839 
Henry Fox Talbot, an English gentleman of high rank, 
succeeded in devising a printing process which produced a 
surface highly sensitive to light. Paper was coated with a 
weak solution of common salt, and a solution of nitrate of 
silver was then brushed over it, the strength of the solutions 
being so adjusted as to leave a slight excess of the silver nitrate. 
Snell paper was a hundred times more sensitive to light than 
paper coated with either the chloride alone or the nitrate alone. 

Let us see what were the chemical changes involved 

AgNOs + NaCl = AgCl + NaNOg 

Silver and Sodium produce Silver and Sodium 

Nitrate Chloride Chloride Nitrate. 


24:8 


THE CHEMISTRY OF PHOTOGRAPHY. 


Tlie common salt (soJium chloride) combines with the silver 
nitrate, and silver chloride is produced. Sodium nitrate is 
also produced; but this may be neglected, as it takes no 
further part in the work. 

It seems strange that the paper coated with silver chloride, 
containing in addition a little silver nitrate^ should be so 
much more sensitive to light than paper coated with the pure 
silver cliloride alone. This is explained by the fact now 
known to us that perhaps no substance is sensitive to light 
when perfectly pure, dry, and isolated. The action of light 
is undoubtedly— -as Scheele proved in 1777 — to separate a part 
(or the whole) of the non-metal (the chlorine in this case) 
from the metallic silver with which it is combined. 

2AgCl = AggCl + Cl 

Silver ChXoxieaQ produces Silver Sub-chloride and Chlorine. 

Let us suppose the above equation to represent the action 
of light upon ordinary white silver chloride. We see that a 
part of the chlorine is driven off, while dark-colored silver 
sub-chloride remains. But unless there be some substance 
present with which the liberated chlorine can combine (as 
water, or, better, silver nitrate) the light is unable to de- 
compose the silver chloride. This can be proved by expos- 
ing dried AgCl to sunlight in a glass tube from whicli air has 
been extracted; its color is unchanged. 

Albumen Introduced in Silver Printing {in 1850) to Im~ 
part a Glossy Surface to the Paper. — This printing method, 
upon paper coated with silver chloride, and with slight excess 
of silver nitrate, which was published by Fox Talbot on 
January 31, 1839, has remained the chief printing ^^rocess in 
use in photography right down to the present day. 

From 1839 to 1850 the paper was always left with its 
natural or “ matt” surface. But in the latter year Blanquard 
Evrard and Gustave Le Gray, in France, introduced the plan 
of giving the paper a preliminary coating of albumen (white 
of egg) by which the surface of the paper was made very 
glossy, the details brought out and the image prevented from 
sinking in. This albumenizcd paper was introduced into 
England by Talbot and Bollock in 1852-3. 


THE CHEMISTRY OF SILVER PRmTING, 


249 


During the last two or three years, however, a strong re- 
action has set in against this inartistic gloss, though for ordi- 
nary portrait work it still maintains its sway. 

Modern Printing in Silver . — The photographic journals of 
the ] 3 resent day usually contain advertisements of album- 
inized paper,” and also of “ ready sensitized ” paper. The first 
of these has to be sensitized b}^ the purchaser — usually the 
professional photographer ; while the latter is preferred by 
the amateur, as it is ready to be at once placed in the printing- 
frame. 

Preparation of Albumenized Paper . — Albumenized paper 
is albumenized and “salted” at the same time. The follow- 
ing mixture is prepared : 


Fresh white of eggs 7 ounces 

Ammonium chloride 75 grains 

Alcohol 2 drams 

Distilled water 2 ounces 


Dissolve the NH^Cl (ammonium chloride) in the alcohol 
and water ; add it to the albumen, and beat up vigorously. 
Filter through cotton -wool which has been steeped in water. 
This quantity should be amply sufficient for a quire of paper. 
Place the mixture in a dish, and float each sheet of jmper upon 
it, in turn, for one and a quarter minutes, taking care to avoid 
bubbles. Then hang the paper up by two corners to dry. 

Double albumenized paper is made by exposing the paper, 
coated as above, to a current of steam by which the albumen is co- 
agulated and hardened. The paper is then floated on a second 
bath of albumen. Such paper has an extremely glossy surface, 
but it is more liable to crack and blister. 

Some commercial makes of albumenized paper have a very 
unj)leasant smell. This is usually due \o fermented albumen 
having been used in their preparation, a method which is in 
favor with some large Arms on the Continent. Such pa]3er wflll 
generally give fine tones. Sometimes, how^ever, the albumen 
used has been allowed to become partially decom2:»osed, or 
putrid, and the smell of sulphuretted hydrogen then produced 
is not only offensive but may be dangerous to the permanence 
of the prints. 


250 


THE CHEMISTEY OF PHOTOGRAPHY. 


Albumen ized paper is made so well (commercially) and sold 
so cheaply that we are not aware that any practical photog- 
rapher has ever found it ‘‘pay’’ to albumenize his own. 

Sensitizing Albumenized Paper. 

A “ bath ” of silver nitrate must be made up by dissolving 
the pure crystallized salt in distilled water. The strength of 
this solution may vary from 30 grains (not less) per ounce for 
hard negatives with strong contrasts, to 80 grains per ounce 
for thin weak negatives. The best average strength is 60 
grains per ounce. 

The quantity of this solution to be made up for use varies 
with different workers, and wdth the size of the pieces to be 
sensitized. When sheets of albumenized paper of the full size 
(17i by 22j inches) are to be floated on the solution, about a 
gallon of it will be necessary. In any case it should be not 
less than half an inch deep in the flat shallow dish usually em- 
ployed. The paper should be kept in a damp place for an 
hour or two before sensitizing ; and it may be floated on the 
silver bath for two minutes (in summer) or three minutes (in 
winter). The chemical change that takes place during the 
floating may be thus expressed : 

AgNOg + NH4CI = AgCl + NH4NO3 

Silver ancf Ammonium produce Silver and Ammonium 

Nitrate Chloride Chloride Nitrate. 

Silver albuminate is also formed ; but its chemical composi- 
tion is very complex and uncertain, and its presence is neither 
desirable nor necessary. 

The pajier is drawn over the edge of the dish to remove the 
otherwise too great excess of the batli solution which would 
cling to it ; and is then either hung up at once by the corners 
to dry, or is first pressed between sheets of chemically pure 
white blotting-paper. 

The worst thing about freshly sensitized paper like this is 
that it discolors if kept for more than two or three days. By 
keeping it between sheets of blotting-paper which have been 
soaked in carbonate of soda solution (1 to 20), and then dried, 
it may be kept wdiite for two or three weeks. But its ad- 


THE CHEMISTRY OF SILVER PRINTING. 


251 


vantages are that it enables black tones to be got more easily, 
and that the paper can be sensitized (by the use of a weak or 
of a strong bath) to suit negatives of varying qualities. 

The silver bath must not be acid. If it turns blue-litmus 
paper red (proving acidity), add a few drops of sodium car- 
bonate solution until the blue color is just restored. 

Ready -Sensitized^^ Paper . — If albumenized sensitized 
paper will not ‘^keep,” the student may inquire how it is that 
paper of this description can be purchased from store-dealers 
which is warranted to keep for several months — sometimes for 
a year? The answer is, that such ‘‘ready-sensitized paper*’ 
has undergone a special treatment. What the exact nature of 
that treatment may be is rigidly preserved as a “ trade secret,” 
and the practice of different firms probably v^aries. 

The following methods for the production of “ ready-sensi- » 
tized paper ” have been published : 

1. Add ten drops of perchloric acid to every ounce of the 
sensitizing bath of silver nitrate. 

2. Or, after removal from the bath, and when the paper has 
become surface-dry, float the hacl>: of the paper for one minute 
upon a solution of citric acid thirty grains to every ounce of 
water. The citric acid combines with the silver nitrate to form 
silver citrate, which a much more stable salt than the nitrate. 

3. Or, similarly, float the pre\flously sensitized paper on the 
following solution : 

Picked while gum arabic, dissolved in six pints of 


water, ... 6 ounces 

Citric acid 2 ounces 

Tartaric acid 2 ounces 

Hydrochloric acid 2 ounces 


Float the hach of the sensitized paper on this mixture for 
from half a minute to flve minutes, according to the length of 
time the paper is required to keep.“ 

4. If the sensitized paper be loashed^ by floating it after sen- 
sitizing and when surface-dry, upon two or three changes of 
distilled water, it will keep for two or three weeks. The 
reason is, that the water removes nearly all the free silver 


*Ashman’s “Lessons in Silver Printing.” 


252 ‘ THE CHEMISTRY OF PHOTOGRAPHY. 

nitrate. But such paper will not tone unless it be fumed 
before printing. 

5. Sensitized paper keeps well if all moist air be excluded. 
This can be done by placing the paper in air-tight tins (like 
those used by the Platinotjpe Co.) containing calcium chloride. 

Beady-sensitized paper is very convenient, and is largely 
used not only by amateurs but b}^ professionals. It does not 
tone so readily, nor are black tones so easily obtained as with 
freshly sensitized paper. Before printing, it should be 
‘"fumed”; or, after printing and before toning, the prints 
should be soaked in a weak alkaline solution (see toning) to 
neutralize the acid by which the paper has been preserved. 

^'Fuming Sensitized Paper P — It is a common practice in 
America — much less so in England — to expose each sheet of 
sensitized paper before printing for about ten minutes to the 
fumes of strong ammonia, placed in a saucer in an air-tight 
box, to the lid of which the paper is pinned. The volatile 
alkali (as ammonia is termed) destroys any free acid which 
may be present in the paper. 

Printing on Matt-Surface Paper . — The reaction against a 
glossy surface has lately led to a return by many workers to 
the practice which was universal before 1852 — the printing in 
silver upon matt ” or “ dead ” surface paper. 

The paper must hOi pure. Especially it must be free from 
chlorine and also from hyposulphite of soda, which is largely 
used by pa23er-makers as an “ anti-chlor ” or substance to 
remove the chlorine. Almost any good white paper will 
answer the purpose. Becent researches have shown that it is 
hardly possible to purchase a sample of paper which does not 
reveal the presence of “ hyj)o ” wlien delicate tests are 
employed. This is much to be regretted. 

The effect of printing upon paper with quite a rough 
surface has, during the last year or two, been much admired. 
Fpr such an object Whatman’s drawing-papers have been 
used, and Mr. Lyonel Clark'^' recommends the paper sold as 
‘‘Arnold’s pure unbleached ” 


■'•'Salting' and Exciting of Drawing and other Commercial Papers : Camera Club Jour- 
nal^ January and November, 1890. 


THE CHEMISTRY OF SILVER PRINTING. 


' 253 


The paper may, or may not, require sizing. Blotting paper 
is unsized, while some varieties of glossy writing paper are 
nearly all sized. The rough paper will certainly need sizing, 
and may be passed through a warm solution of gelatine of the 
strength of from 12 to 24 grains per ounce of water. It must 
then be hung up to dry. 

The next thing to do is to salt the paper. For this purpose 
float the dry paper for three minutes on — 


Ammonium chloride 130 grains 

Sodium carbonate 3 grains 

Water 1 pint 


Hang up to dry in a warm room. 

Sensitizing with Ammonio- Nitrate of Silver. — To sensitize 
this paper we may use the ammonio-nitrate bath, first recom- 
mended by Dr. Taylor in 1841, and improved by T. F. Flard- 
wich in 1855. It is especially useful for weak negatives and 
for printing in dull vmather. 

Dissolve 60 grains of silver nitrate in half an ounce of dis- 
tilled water. To this add ammonia, drop by drop, until the 
black precipitate first formed is just redissolved. The liquid 
should be stirred continually with a glass rod. 

Divide the solution into two parts, and to one part add nitric 
acid, drop by drop, until the color of blue litmus is just 
changed to red. Then mix the two parts together, and add 
enough water to make up to 1 ounce. Filter, if not perfectly 
clear. 

How to Apply the Ammonio- N itr ate Solution. — It is best 
to apply this sensitizing solution with a hrush. A camel-hair 
brush may be used, but it soon becomes spoiled and useless. 
The best method is to use a ‘‘ Blanchard’s Brush,” named after 
its inventor, the well-known English professional, Yalentine 
Blanchard. It is made by folding a double thickness of 
swan’s-down calico over the end of a strip of glass from three 
to six inches wide. The calico must be tied on, or secured to 
the glass by a rubber band. A pool of the sensitizing solution 
is poured upon one end of the sheet of paper (laid on a flat 
surface, as a sheet of glass), and this is led over the paper with 
the brush. The paper should be thoroughly moistened in 


254 : 


THE CHEMISTRY OF PHOTOGRAPHY. 


everj part with the solution. It may lie for a minute to allow 
the solution to soak in, and should then be hung up near a fire 
to dry. 

This paper will keep for a week or two if preserved between 
sheets of soda blotting paper. 

Action of Light upon Sensitized Paper . — When ‘‘sensi- 
tized paper ” is spoken of in photography, the ordinary albu- 
menized paper containing chloride of silver (plus a little 
nitrate) is always meant. 

When such paper is exposed to light its white surface is 
gradually changed in hue, passing through various shades of 
brown, gray, and violet, to a brown or violet black. 

What is the precise chemical change produced by the action 
of light upon the silver chloride ? That has long been — and 
still remains — one of the puzzles of photographic chemistry. 
The subject is treated of in greater detail in discussing the 
‘4atent image”; but it will suffice here to say that in the 
present state of our knowledge the following equation repre- 
sents more nearly than any other the probable facts : 

2AgCl == iAgsCl 

Silver chloride, when decomposed by light, produces silver sub-chloride 

+ Cl 

and chlorine. 

Chlorine is undoubtedly given ofi — Scheele proved that in 
1777 — but whether it is entirely or only in part separated from 
the silver is still doubtful. 

Printing-Papers with Glossy Surfaces Obtained Other- 
wise THAN BY Albumen. 

After the introduction of albumen to give a surface gloss to 
prints about the year 1850, the desire for a highly-polished 
surface (mainly, and especially for portrait work) so increased 
that many endeavors were made to satisfy it. 

The employment of (^o^^^Z6-albunlenized paper we have 
already mentioned. There is no doubt but that the second 
coating with albumen enables a superior degree of glossiness 
to be obtained, and that this gloss does throw up and relieve 
the shadows, and brings out the details. 


THE CHEMISTRY OF SILVER PRINTING. 


255 


Blanquart-Evrard Yarnishes Prints with Gelatine (1857). 
— The great French professional printer, M. Blanquart-Evrard, 
of Lille, proposed, in 1857, to protect and strengthen prints 
bj a varnish composed of gelatine and tannic acid. A refer- 
ence to this method in the Photographic News for 27th May, 
1858, called forth a letter from an English worker, Mr. W. L. 
Scott (J line 17), in which he stated that he had practised such 
a process for two or three years. The prints were dipped in 
a warm solution of pure gelatine, dried, and then soaked in a 
colorless solution of tannic acid (200 grains to the pint) for 
ten minutes. After drying, they were immersed in the same 
solutions over again, and finally rinsed and dried. This 
process gave a high gloss to the }3aper, and was believed to 
render the prints more permanent. The action of the tannin 
is, of course, to harden the gelatine — to convert it into a sort 
of transparent leather, in fact. 

Burnishing and Rolling Prints. — The use of a “flat-iron” 
to level the surface of a mounted print may not impossibly 
have occurred to some worker of Talbot’s “photogenic” proc- 
ess, even as early as 1839. Nay, it is possible that the advan- 
tage of using the said flat-iron hot instead of cold may have 
been discovered at quite as early a date ; but this genius, 
strange to say, did not patent his “ application ” of the useful 
domestic implement to this purpose of high art, and conse- 
quently his name has not come down to us. 

Up to the year 1858 portraits were all but invariably made 
by professional photographers, either upon silver plates 
(daguerreotypes) or with collodion upon glass. In either way, 
the finished portrait was placed in a suitable case — a gjasse- 
partouh ^ frame — before being delivered to the customer ; 

indeed its delicate nature made this inevitable. 

But in 1858 the mania for the carte-de-visite sprang up. 
Everybody desired to present his or her card-portrait to every- 
body else ; and the professionals reaped a golden harvest for 
several years. But wdth the advent of the positive paper-print 
stuck upon cardboard, the flat-iron came out in great force 
again. Then it quickly dawmed on some inventive genius 
that, by passing the mounted prints between steel rollers., the 


256 


THE CHEMISTRY OF PHOTOGRAPHY. 


flattening and smootliing would be rapidly and eflectually 
accomplished. Lastly, it was found that if a hot steel bar or 
plate were substituted for the lower roller, the prints were 
huryiished to a degree that gave them a surface almost equal- 
ing glass ; and such instruments — called ^^burnishers’’ — have 
proved all but indispensable to the professional portraitist ever 
since. 

Printing in “ Wothlytype.” 

The name of Wothly (or Wothlij) is unknown to the pres- 
ent generation of photographers ; but twenty-seven years ago 
his printing process created quite a sensation, and the ‘‘ United 
Association of Photography,” with Colonel Stuart Wortley at 
its head, was formed to purchase the patent and to work it 
commercially in England. The English patent itself is dated 
24th September, 1864. Paper was sized with arrowroot and 
then rolled. It was then coated, of course in a dark-room, 
with collodion in which silver nitrate and uranium nitrate had 
been dissolved. The paper having been dried, was printed 
out beneath a negative in the usual way. It was then toned 
and flxed as usual. 

All sorts of foolish claims were made for Wothlytype. It 
was puffed in the Times in the autumn of 1864, and was said 
to be very cheap and capable of giving permanent results. 
As a matter of fact, the patent contained little that was new. 
In 1857, the Scotchman, Burnett, had described all the facts 
about uranium printing. The truth is that M. Wothly was an 
excellent operator and a good man of business. He produced 
first class negatives and made exquisite prints from them. He 
sold his patent ; but he could not sell the skill to which — and 
not to the patent — the production of his capital specimens ” 
was due. AVothlytype ran but a brief race; after a year or 
two nothing more was heard of it. But it doubtless furnished 
Simpson with the idea of the collodio-chloride printing proc- 
ess, which we shall next describe. 

Printing with Collodio-Chloride of Silver : Simpsontype. 

At the close of the year 1864, Mr. G. AVharton Simpson 
(then editor of the Photographic News) announced in the 


THE CHEMISTRY OF SILVER PRINTING. 


257 


‘‘Year Book” or almanac connected with tlie same periodical, 
that lie had “discovered that chloride of silver may be held 
in suspension in collodion in a state of subdivision so exceed- 
ingly tine that it may be used in this form for coating paper, 
and gives then, with the usual manipulation, exceedingly tine 
prints, in which, when finished, no silver is found in the 
whites P The process was developed and jierfected during 
1865 and 1866; but although beautiful results were obtained 
— especially upon opal glass — the collodio-chloride printing 
process never came into general use. 

In practice the paper w^as first sized with arrowroot to pre- 
vent the emulsion from sinking in. Chloride of silver was 
then formed in collodion, by shaking up in it nitrate of silver 
with chloride of strontium. 

2AgN03 + SrCIg = 2AgCl + SrtNOg)^ 

Silver Strontium Silver Strontium 

Nitrate and Chloride produce Chloride and Nitrate 

The paper was coated by laying it upon a flat surface, turn- 
ing up its edges all round, but leaving a corner from which to 
pour ; the emulsion was then poured on and off just as in 
coating a glass plate. The paper so prepared was dried, and 
then printed-out, toned, and fixed in the usual way. 

Collodio-chloride pa^^er was manufactured, commercially, 
on the Continent, by Herr Obernetter. 

It is unrivalled for the delicacy of the detail which it brings 
out ; and is specially suited for printing from thin and weak 
negatives. 

It was also made and sold by another Continental firm under 
the name of “ leptographic pa]3er.” 

Aristotype and Ohernetter Papers , — During the last two or 
three years collodio-chloride has again been resuscitated ; and 
has been sold as “aristotype” and “Obernetter” paper. 
But other papers coated with gelatino-oh\oY\^Q of silver have 
also been sold under these names, and it is a matter of some 
importance to be able to distinguish between them. This 
may be effected by treating 2 ^ print with wood-naphtha, which 
will dissolve collodion, but which has no effect upon gelatine. 


CHAPTER XXY. 


THE CARBON PRINTING PROCESS AND ITS 
CHEMISTRY. 

Permanence of Carhon. — The element whose proper name 
is carhon, but which assumes such different forms as the dia- 
mond, coke, lamp-black, charcoal, etc. (each and all of which 
are composed of nothing but carbon), is perhaps the most 
2 )ermanent^ under ordinary conditions, of all the substances 
with which chemistry has made us acquainted. In the form 
of “ printer’s ink,” carbon assures the permanence of books, 
engravings, etc.; and in the ancient papyri of Egypt we have 
manuscripts written in carbon which are as easy to decipher 
now as they were thirty centuries ago. Xo wonder that pho- 
tographers in their search for a permanent printing process 
turned their eyes longingly to carbon early in the history of 
the photographic art. 

Ponton Discovers the Action of Light on Bichromate of 
Potash. — In 1839, Mungo Ponton, a Scotch experimenter, 
announced * the fact that paper coated with a solution of 
potassium bichromate was turned brown by exposure to light. 
Any one can repeat this experiment by floating writing paper 
upon a 10 per cent, solution of the bichromate, and then 
exposing the dried paper to sunlight beneath a negative or an 
engraving. Such a print is fixed” by simple washing in 
water, which removes the unaltered and still soluble bichro- 
mate. 

Becquerel Shows that a Colloid must he Present. — In 
repeating Ponton’s experiment the great French chemist, E. 
Becquerel, found that the size in the paper played an impor- 
tant part in the reaction. 

Talhot Discovers that a Mixture of Gelatine and Potash 
Bichromate is Rendered Insoluble by Exposure to Light . — 


* Edinburgh New Philosophical Journal^ Vol. XXVII,, pp. 1G9-171. 


THE CARBON PRINTING PROCESS AND ITS CHEMISTRY. 259 

Unless some colloid body (as glue, gelatine, starch, etc.) be 
present, the bichromate of potash is not affected by light. But 
Talbot found that when the bichromate was mixed with some 
colloid (the ‘‘ size ” in paper is only weak glue) the effect of 
exposure to light was not merely a change of color, but the 
colloid body was rendered insoluble in liquids in which it had 
previously been soluble. This important fact was discovered 
by Henry Fox Talbot in 1852, and was patented by him as 
part of a photo-mechanical printing process which he called 
“ photoglyphic engraving,” on October 29th in that year. 

Poitevin^ Sutton^ and Pouncy obtain Carbon Prints. — The 
French chemist, A. Poitevin, in 1855 added powdered carbon 
to Talbot’s mixture of bichromate and glue (or other colloid). 
Paper was coated with this mixture, dried, exjDosed to light 
beneath a negative, and then washed in warm water. The 
glue, etc., unaffected by light was dissolved away, leaving the 
insoluble glue (holding carbon, and therefore colored) to form 
a positive picture. In England, Thomas Sutton and John 
Pouncy discovered the same process, independently, in 1858 ; 
and long accounts of it were printed in the periodical edited 
by Sutton, Photographic Notes., in 1858-59 ; and also in a 
little book, Photography in Printing Ink,” which Sutton 
wrote in 1863. The black color of printing ink is, of course, 
due to finely-divided carbon. 

Half-Tones wanting in the Tarty Carbon Prints. — The 
light, acting through the negative, affected the surface of the 
carbon print beneath. In the deepest shadows of the picture 
(represented by clear glass in the negative) the light had time 
to render the carbon tissue beneath, insoluble right down to 
the paper backing. Under the high-lights ” (represented 
by a dense and opaque deposit of silver in the negative) the 
tissue is quite unaffected and remains soluble. But under the 
half-tones ” of the negative the tissue is affected to depths 
varying with the opacity of the deposit of silver representing 
tlie half-tones. When the tissue is removed, and its surface 
washed, the layer of soluble gelatine which remains beneath 


* The thin paper coated with a mixture of gelatine, bichromate of potash, and powdered 
carbon, is called “ carbon tissue.” 


260 THE CHEMISTRY OF PHOTOORAPHY. 

the insoluble surface parts, representing the half-tones, is dis- 
solved away, and it usually carries aioay the ujypeT layer with 
it. Thus only a hard, black-and-white carbon picture remains. 
This fact was clearly pointed out by the Abbe Laborde in a 
communication relating to an analogous process made to the 
French Photographic Society, in 1858. 

Half-Tones secured hy Burnett {1858) and ly Fargier 
{I860), — The Scottish experimenter, J. C. Burnett, proposed* 
in 1858 the remedy of placing the hach, or uncoated side of 
the paper next the negative; but this was impracticable, 
because of the very long exposure thereby rendered neces- 
sary ; and because the texture of the paper was imparted to 
the print. 

The real remedy for the lack of half-tones in carbon print- 
ing was patented by a Frenchman named Fargier, in Septem- 
ber, 1860. It consisted in stripping off the paper back of 
the tissue, and then applying the solvent, the warm water, 
to the hack of the carbon film. To do this, it was nec- 
essary to strengthen the film by a coating of collodion 
applied to its face. Good prints in carbon now became pos- 
sible; but the manipulations under Fargier’s method were 
very difficult. 

Swa?i, Johnson and Sawyer make Carhon Printing a 
Practical Success. — The patent of the English worker, J. W. 
Swan, dated 28th February, 1864:, for the first time put a 
really practical, successful, and comparatively easy means of 
producing carbon prints before the photographic world. And 
yet Swan’s improvements may be considered as only ‘details”; 
but it is just these details which make all the difference 
between failure and success. lie mixed a little sugar with 
the gelatine to render it less brittle when dry. After expo 
sure beneath a negative, the print was stuck, face down, on 
either a temporary or a permanent support ; and the paper 
backing, with the soluble gelatine beneath it, was washed 
away with warm water. The picture was thereby “devel- 
oped ” — or rather made visible. But by the single transfer it 
was, of course, reversed. In some cases this reversal does not 


* Photographic Journal for 22d November, 1858. 


'JHE CARBON PRINTING PROCESS AND ITS CHEMISTRY. 261 

I 

matter ; but, usually, it is necessary to again transfer (‘‘double 
transfer”) the carbon print to a second and permanent sup- 
port or backing. Or, if a reversed negative be made to begin 
with, by placing a prism, or a mirror, in front of the lens, 
then the single transfer only is necessary. Swan also intro- 
duced many other powdered colors, as red chalk, etc., in place 
of carbon ; so that pictures in any tint could be obtained. 
But of course these lacked the permanence which is the great 
recommendation of carbon. Swan used many adhesives to 
make the carbon tissue adhere to its various “supports”; but 
in 1869 J. R. Johnson showed that it was only necessary to 
first soak the carbon tissue in water for a short time, in order 
to enable it to adhere to any water-proof support. Lastly, in 
1874,- J. R. Sawyer patented a “flexible support,” consisting 
of water-proof waxed paper, which most conveniently sup- 
ported the tissue while it was being developed. 

In the following year — 1875 — two French photographers 
who jDossessed excellent powers of manipulation, exhibited the 
carbon process in pra^ctice in most large towns in England and 
on the Continent ; and succeeded at last in drawing general 
attention to its many excellent points. It was then thought 
that carbon printing would displace silver ; but the idea has 
proved fallacious. The glossy silver print has held its own ; 
though there are not now wanting signs which seem to show 
that its reign may not be of much longer duration. The 
Autotype Company, of London, established by Swan and his 
partners, has done much for the advancement of carbon print- 
ing ; while on the Continent a similar good work has been 
performed by the firm of A. Braun, of Dornach. 

Practical Carbon Printing . — Just as in silver-printing, the 
carbon tissue can be bought either sensitized or unsensitized. 
In appearance it resembles black American oil-cloth. The 
plain or unsensitized tissue consists of paper coated with a 
solution of gelatine and sugar, to which refined lamp-black 
has been added. 

The bichromate of potash — which is the sensitizing ingre- 
dient — can either be added to the above substances before 
coating, or the coated paper may be sensitized by floating it 


262 


THE CHEMISTRY OF PHOTOGRAPHY. 


upon a 4 per cent, solution of the bichromate, to which a little 
ammonia has been added. After sensitizing, the carbon tissue 
will not keep good for more than ten or fourteen days. 

The black tissue is exposed to sunlight beneath a negative 
in the usual way. The negative must have a “ safe-edge ’’ 
about the eighth of an inch wide, painted all round it in any 
opaque black varnish. This is to insure the adhesion of the 
margins of the tissue to the support during development. 
Tlie ordinary ready-sensitized carbon tissue is a little more 
rapid than ordinary albumenized paper. If, therefore, it be 
printed along with the latter, each under a negative of average 
density, when the one is done the other will be done. A 
special instrument, called an actinometer, is generally used to 
determine the time of printing. 

Four dishes are necessary for development. The first con- 
tains cold water and a piece of waxed ^^fiexible temporary 
support.” The exposed carbon tissue is soaked in cold water 
for a couple of minutes, and is then squeegeed down upon the 
support. It is placed between blotting-paper, and left under 
gentle jiressure for twenty minutes. After this space of time 
the carbon tissue is placed in a second dish containing water, 
at 100 deg. F. In a minute or two the paper backing may be 
stripped off, and by dashing the warm water upon the print 
the still soluble part of the gelatine may be washed away, and 
the picture revealed. The print is then washed in cold water 
in a third dish. The fourth (and last) dish contains a satu- 
rated solution of common alum. The now developed print is 
soaked in this till all the yellow tint (due to the bichromate) 
has disappeared. It is then washed in several changes of 
plain water to get rid of the alum. The final operation 
consists in squeegeeing a piece of permanent support ” 
(paper coated with soluble gelatine) upon the print, which is 
then allowed to dry. As the carbon print dries it separates 
itself from the waxed surface of the “ temporary support 
but adheres firmly to the permanent support.” It may then 
be trimmed and mounted in the ordinary way. 

All the ‘^supports” and other materials named are pre- 
pared commercially, of great excellence and moderate in price. 


THE CARBON PRINTING PROCESS AND ITS CHEMISTRY. 263 

It is far better to purchase them than to attempt to make 
them, except for the sake of experiment. 

Chemistry of the Carhon Printing Process . — In the paper 
by Mungo Ponton, already alluded to, and which he published 
in 1839, he describes clearly and forcibly the effect of light 
upon potassium bichromate. Ponton writes : — “ Paper im- 
mersed in bicliromate of potash is powerfully and rapidly 
acted upon by the sun’s rays. * * * When an object is 

laid in the usual way on this paper, the portion exposed to the 
light speedily becomes tawny, passing more or less into a deep 
orange, according to the strength of the solution and the 
intensity of the light. The portion covered by the object 
retains the original bright yellow tint which it had before ex- 
posure, and the object is thus represented yellow upon an 
orange ground, there being several gradations of shade or tint, 
according to the greater or less degree of transparency in the 
different parts of the object. 

In this state, of course, the drawing, though very beauti- 
ful, is evanescent. To fix it, all that is required is careful im- 
mersion in water, when it will be found that those portions of 
the salt which have not been acted on by the light are readily 
dissolved out, while those which have been exposed to the 
light are completely fixed on the paper. By this second 
process the object is obtained white upon an orange ground, 
and quite permanent.” 

Ponton’s bichromate pictures may have appeared “ beauti- 
ful ” to his astonished eyes, but it is to be feared that they 
would not gain many admirers now-a-days. 

In the presence of some organic material, as the fibre of 
paper, the size with which the paper is usually coated, etc., 
bichromate of potash undergoes the following decomposition 
when exposed to light : 

KgCroO, KaCrO^ + CrOg 

Bichromate of potash produces Chromate of Potash and Chromic Acid. 

The chromic acid is then further decomposed as follows : 

CrOg = CrOo + O 

Chromic Acid produces Chromium Peroxide and Oxygen. 


264 


THE CHEMISTEY OF PHOTOGKAPHY. 


The chromium peroxide is of a tawny color, and — by its 
contrast with the bright yellow bichromate — produces the pic- 
ture. But by prolonged exposure to light, the chromium 
peroxide loses another atom of oxygen, and becomes reduced 
to chromium sesquioxide, thus : 

2Cr03 = CrgOg + O 

Chromium produces Chromium and Oxygen. 

Peroxide Sesquioxide 


This chromium sesquioxide is of a greenish tint, and the 
contrast which it produces wdth the bichromate is not so 
marked ; hence by long exposure the picture becomes weaker. 

ISTow what becomes of the oxygen which is liberated? It 
combines with any colloid substance (as gelatine) which may 
be present, and renders it insoluble. This was very clearly 
explained by Poitevin in a book* which he published in 1 862 : 
The chromic acid loses (by exposure to light) a part of its 
oxygen, which combines with the organic matter and renders 
it insoluble. When the film is washed, the carbon remains 
adhering to the exposed insoluble parts, and forms the pic- 
ture.” As to the precise nature of the oxidized gelatine 
])roduct formed, most of what we know is due to tlie researches 
of Dr. Eder, published f in 1878; but the subject is a difficult 
and obscure one. Captain Abney gives the following equation 
(which we have simplified) as representing the final action of 
the bichromates upon organic matter generally : 


Cx Hy Oz + 

Organic and 

Matter 

CrgOg 

Chromium 

Sesquioxide 


KsCrgO^ = 2KHO + 
Potassium produce Potassium and 
Bichromate Hydrate 

+ Cx Hy Oz OO 

and Oxidized 

Organic Matter. 


In this equation the letters x, y, and z are used simply to 
denote indefinite quantities of each element. The oxidized 
organic matter (gelatine, etc.) is found to be insoluble in 
liquids in which the ordinary organic matter is quite soluble. 

* “ L’impression photographique sans sets d’argent.” Paris : Leiber, 1862. 
t “ Ueber die Reactioner der Chromstiure und der Chromate auf Gelatin, Gummi, 
Zucker, etc.” V/ien, 1878. 


THE CARBON PRINTING PROCESS AND ITS CHEMISTRY. 265 


Dr. Paul E. Liesegang has written an excellent Manual of 
the Carbon Process,”'^ which should be studied by all who 
desire to practice this excellent and permanent method of 
photographic printing. 

* Translated from the German, and sold by The Scovill & Adams Co. 



CHAPTER XXYL 


PRINTING WITH SALTS OF IRON— CYANOT YPE AND 

KALLITYPE. 

Ilerschel Puhlishes the Cyanotype Process in 1842. — In a 
valuable paper entitled On the Action of the Rays of the 
Solar Spectrum on Vegetable Colors, and on some new Photo- 
graphic Processes,” written by Sir John E. W. Herschel, and 
published in the Philosophical Transactions for the year 
1842, we find the common “blue process” of the present day 
described under the name of cyanotype. The process appears 
to have “sprung full-fiedged” from Herschel’s brain; for the 
exact method he gives will produce excellent results, and has 
been little varied since. It is often called the “ ferro-prussiate 
process,” from the names of the two chemicals which are em- 
ployed in it. 

Cyanotype in Practice. — The “ blue process,” or cyanotype, 
deserves to be more widely known and practiced than at 
present. It is more favored in America than in England. It 
is very cheap, very clean, easy to work, and the results are per- 
manent. The blue color suits many subjects admirably. The 
paper to be used should be well sized, in order to keep the 
chemicals as far as possible on the surface ; otherwise the pic- 
ture has a dark and sunken-in appearance. Highly-sized white 
note-paper answers well. Or any paper can be sized by 
making arrowroot into starch and sponging it over the paper 
to be used, which must then be dried. If ordinary albumen- 
ized (not sensitized) paper be soaked for a minute in boiling 
water, to coagulate the albumen, it will yield very brilliant 
blue prints. 

Make up the following solutions : 

No. 1. 


Ammonio-citrate of iron 1 ounce 

Distilled water 4 ounces 

No. 2. 

Red prussiate of potash 1 ounce 

Distilled water 4 ounces 


PRINTING WITH SALTS OF IRON. 


267 


These solutions must he kept in separate bottles, which 
should have brown paper glued round them, to protect the 
contents from the light. 

Ammonio-citrate of iron is sold at most druggists’ shops as 
citrate of iron and ammonia.” Its chemical formula is 

(C^H^OdsFe, (NHP3 

The red prussiate of potash is more properly named ferrid- 
cyanide of potassium” — Kg FeCy^. 

Mix the solutions 1 and 2 in equal proportions in a clean 
glass dish, and add for each ounce of the mixture 5 dro^^s of a 
10 ]3er cent, solution of ammonium bromide. Mix well by 
stirring with a glass rod. The liquid so prepared is sensitive 
to light, and the operation of coating the paper to be used 
should be done by gas-light or in a dark corner of a room. 
The paper to be sensitized may be floated upon or soaked in 
the solution for two or three minutes ; when lifted out it should 
be drawn over a glass rod to remove the excess of the liquid. 
The paper may also be laid upon a sheet of glass or a board, 
and the mixture applied to its surface by means of a clean 
sponge. In any case the paper should be dried in a dark room 
near the Are. The sooner it is used the better ; for although 
cyanotype paper will keep fairly well for days, or even weeks, 
it never gives such bright blue tints as when just freshly 
prepared. 

Cyanotype jiaper is printed beneath a negative in the usual 
way ; it takes two or three times as long to jirint as ordinary 
silvered paper. When done, the picture can be plainly seen 
in brown and yellow, the shadows being bronzed. Kow re- 
move the print from the printing frame and immerse it in 
water, to which a little hydrochloric (or citric) acid has been 
added (just enough to make it taste sour). Finally wash in 
flve or six changes of plain water. The result should be 
a brilliant print in blue lines upon a white ground. 

Chemistry of the Cyanotype Process. 

It is easy to reduce the ferric compounds (or ^‘per-salts of 
iron,” as they used to be called) to the ferrous state proto- 


268 


THE CHEMISTRY OF PHOTOOEAPHY. 


salts”) by chemical means alone. Thus, nascent hydrogen is 
capable of effecting this change, converting ferric sulphate into 
ferrous sulphate. 

Fe 2 (S 04)3 + Hg = 2 FeS 04 + 

Ferric Sulphate and Hydrogen produce Ferrous Sulphate and 

HgS 04 

Sulphuric Acid. 

Light is also capable of effecting such a change in ferric 
compounds ; but there must be some substance present, some 
“ sensitizer,” which is capable of combining with the oxygen 
or other non-metallic substance given off by the ferric salt. 
Take ferric chloride, Fe^Clg ; light has no effect upon this 
substance when simply dissolved in water, because the water is 
incapable of combining with the chlorine. But when ferric 
chloride is dissolved in alcohol and exposed to light, the follow- 
ing change takes place : 

FcgClg + CgUgO = 2FeClg 

Ferric Chloride and Alcohol produce Ferrous Chloride and 
C 3 H 4 O + 2HC1 

Aldehyde and Hydrochloric Acid. 

In the ordinary ^‘ferro-prussiate paper,” the paper itself and 
the ^‘size” with which it is coated are able to act as sensitizers. 
The paper is coated with ammonio-citrate of iron (though 
ferric chloride and other ferric salts will answer). On expo- 
sure to light the iron salt is reduced to a ferrous state — some 
of its oxygen, etc., being removed — though the precise com- 
position of the substances formed is hardly known with cer- 
tainty ; but that is immaterial. The main point to remember 
is that light chamjes (when a suitable “ sensitizer ” or halogen 
absorber is present ) salts into ferrous salts. 

The advantage of the change, plvotographically speaking, is 
this : Ferric salts are unaltered when mixed with red prussiate 
of potash ; ferrous salts form a blue precipitate with the same 
substance. 

GFeClg -t- 4 K 3 FeCye = 

Ferrous Chloride and Potassium Ferridcyanide p7vduce 

2Fe3(FeCyJg + 12KC1 

Ferrous Ferridcyanide and Potassium Chloride. 


PRINTING WITH SALTS OF IRON. 


269 


The ferrous ferridcyanide is a fine blue solid, long known in 
commerce as ‘‘ Turnb nil’s Blue,” and used as a paint. 

Thus light, acting through a negative upon the ferro- 
prussiate paper beneath, converts the ammonio-citrate of iron 
into a ferrous salt, more or less completely according to the 
relative transparency of the different parts of the negative. 
Under the opaque parts no change takes place. By floating 
upon water, the substances with which the paper is coated are 
all brought into solution, and they then act chemically upon 
one another, with the result that a picture in blue lines upon a 
white ground is produced in the way described above. 

It is quite possible to coat the paper with the ammonio- 
citrate of iron only; and then, after exposure to light, to 
develop it by floating upon a solution of the red prussiate of 
potash. 

The Kallitype Printing Process — Kallitype No. I. 

The “ kallitype ” process takes its name from the same two 
Greek words, signifying “ beautiful picture,” from which Fox 
Talbot derived the name of his calotype ” negative process, 
patented by him in 1841. As the two words sound very simi- 
larly they are liable to be confounded, and it seems a pity that 
some more distinctive name was not chosen. 

“ Kallitype” was patented in 1890 by the inventor. Dr. W. 
W. J. Nicol, lecturer on chemistry at the Mason College, Bir- 
mingham, the number of the specification being 5,374. (Feb- 
ruary 15, 1890.) 

The principle of kallitype consists in exposing to sunlight, 
beneath a negative, paper coated with ferric oxalate. The 
action of lii^ht is to reduce this substance to ferrous oxalate : 

O 

Fe2(C20P3 = 2Fe(C20p + 200^ 

Ferric Oxalate produces Ferrous Oxalate and Carbonic Acid Gas. 

The exposed paper is then developed by floating it for fif- 
teen seconds upon the following solution, used cold : 


Nitrate of silver 50 grains 

Citrate of soda 1 ounce 

Bichromate of potash 1 grain 

Water 10 ounces 

Strong ammonia 34 drachm 


270 


THE CHEMISTRY OF PHOTOGRAPHY. 


To prepare this developer, dissolve the silver nitrate in about 
1 ounce of the water, and the soda and potash in the remain- 
der, and mix. Then add the ammonia and filter. 

The chemical action of this developer can hardly be repre- 
sented by equations ; but it is plain that the ferrous oxide con- 
tained in the ferrous oxalate reduces the silver oxide in the 
silver salt to the state of metallic silver. 

2FeO + AggO = 2Ag + Fe^Oa 

Ferrous Oxide and Silver Oxide produce Silver and Ferric Oxide 

The object of the citrate of soda in this and in the washing 
solutions is to prevent the precipitation of the iron by the am- 
monia used for dissolving the silver salts. 

It now only remains to wash everything out of the paper 
except the black metallic silver which forms the picture. This 
is effected by soaking the print for ten minutes in each of the 
following three solutions : 

Washing Solution No. 1. 


Kallitype developer 34 ounce 

Citrate of soda (pure, neutral) 2 ounces 

Water 20 ounces 

Washing Solution for Baths Nos. 2 and 8. 

Citrate of soda (pure, neutral) 1 drachm 

Ammonia (.880) 2 drachms 

Water 1 quart 


These two baths must always smell distinctly of ammonia. 

Finally the prints are rinsed in several changes of water and 
then dried. 

All the solutions can be bottled and used over and over 
again. The paper was sold by the Birmingham Photographic 
(3o., Gladstone Road, Birmingham, at ten pence per sheet (26 
x 20 inches), so that the process was a cheap one. It gives 
prints of brown or black tones, not unlike bromide paper or 
platinotype ; and as no hypo is employed for fixing, the prints 
should be more permanent than ordinary silver prints. 

The ]>rinting under the negative must be carried on until a 
faint brown image is visible, just showing the details under 
the densest parts ; this only requires about ten minutes in dif- 


PRINTING WITH SALTS OF IRON. 


271 


fused light, or two minutes in sunshine. After floating on the 
developing solution, it is a good plan to lay the prints, face 
upward, on a sheet of clean glass for a minute or two, when 
they will gain in brilliancy and in depth. 

Kallitype No. II. 

In 1891 Dr. Nicol improved his kallitype printing process 
by putting the silver salt in the ]ja])e7\ The new paper is 
coated with two iron salts — ferric oxalate and ferric nitrate — 
and also with the corresponding two silver salts — silver oxalate 
and silver nitrate. By exposure to light the ferric oxalate is 
reduced to the ferrous state. 

The print is then developed by floating upon the following 


bath ; 

Rochelle salt (NaKC4H406) 1 ounce 

Borax ^ ounce 

Water 10 ounces 


Add to this 10 drops of a solution of bichromate of potash. 
(Strength, 20 grains to 1 ounce.) 

This gives black tones, wdiich can be changed to purple by 
diminishing the borax to one-quarter of an ounce. 

The ferrous oxalate combines with the Bochelle salt, and 
reduces the silver to the metallic state ; the Bochelle salt also 
combines with the iron to form ferric tartrate — Fe^ (C4 
^4^6)3* 

The prints should be left in the developing bath for at least 
twenty minutes. They are then removed and fixed by immer- 
sion in two baths of water to which ammonia has been added 
in the proportion of four drachms to every quart. 


CHAPTER XXYII. 


THE PLATINOTYPE PRINTING PROCESS AND ITS 

CHEMISTRY. 

Certain compounds containing platinum have long been 
known to be somewhat sensitive to light ; but it may be at 
once said that in the platinotype process the effect of light 
upon platinum compounds may be altogether neglected. The 
process is, in fact, an indirect one. YYe get the light to act 
upon a certain salt of iron, which is mixed with a platinum 
salt, and then the altered iron salt is caused to act chemically 
upon the platinum salt. 

A jpermanent printing process has always been a great 
desideratum in photography. No “silver print” can be con- 
sidered permanent ; although there are exceptions which 
prove the rule,” yet it is a well-known fact that the great 
majority of ordinary photographs printed in silver — upon 
glossy album enized paper — deteriorate steadily from the time 
of their production until they become yellow and faint, 
perhaps even disappearing altogether. 

Now there are two substances known to the chemist, whose 
permanence he regards as “ beyond reproach ” ; these are car- 
bon and platinum. 

The carbon process has been practised since 1858, and will 
be treated of separately. Platinum had also been used to 
‘‘tone” prints, etc., but until William Willis, Jr., announced 
his results in 1873, no one had succeeded in obtaining a good 
photographic print in metallic platinum. Willis improved his 
process, and took out further patents in 1878, 1880, and later 
years. We shall not follow all the steps which led Willis to a 
final and great success, but will describe, from a chemical point 
of view, the perfected platinotype process as now practised. 
In England, at all events, platinotype is now the process 
employed by the majority of the best workers when they wish 
to obtain the best results. 


THE PLATINOTYPE POINTING PKOCESS, ETC. 273 

Table of the Platinotype Processes, Showing the 
Y AR ious Modific ations. 

Hot-Bath Platinotype, Willis, 1873 (perfected 1880). 
Cold-Bath Platinotype, Ho. I., Willis, 1888. 

Printing-Out Platinotype, Pizzighelli, 1888. 

Cold-Bath Platinotype, Ho. II., Willis, 1892. 


Platinum forms two series of compounds with the non- 
metallic elements. Thus, taking chlorine as a type of the 
non-metals, we have Platinic Chloride, Pt CI 4 ; and Platinous 
Chloride, Pt Cl 3 . Willis reasoned that it would be better to 
employ the latter or -ous series, since there would be less work 
to be done in separating platinum from two atoms of chlorine 
than from four atoms. This was the first element in his suc- 
cess. The experimenters before him had used the higher or 
-ic series. 

Willis’ second discovery was that when ferrous oxalate is 
dissolved in neutral potash oxalate it is able to instantly 
reduce to the metallic state the platinous salts mentioned 
above. 

ferrous oxalate is produced whenever oxalate is 
exposed to light, the change which takes place being .expressed 
chemically as follows : 

Fea(C 204)3 = 2 Fe(Co 04 ) + 2CO^ 

Ferric oxalate becomes Ferrous oxalate and Carbonic acid gas. 

Coat some paper with a solution of ferric oxalate (100 
grains to the ounce of water) ; dry, and expose to light 
beneath a negative. A brownish image will be formed, which 
consists of ferrous oxalate. By itself this image is of no use, 
but it can be used to produce an image in metallic platinum. 

Select some strong, smooth white paper (the best kind 
of drawing-paper, for example). Size this paper by dipping 
it into a weak solution of gelatine (150 grains to the ounce of 
water), the object of the sizing being to prevent the chemicals 
with which the paper is to be coated from sinking too deeply 
into its substance. 


274 


THE CHEMISTKY OF PHOTOGRAPHY. 


The Hot-Bath Platinotype Process. 

For coating the paper two solutions must be prepared : 


1. Ferric oxalate 120 grains 

Oxalic acid (crystallized) 6 grains 

Water (distilled) 1 ounce 


2. Chloro-platinite of potassium solution. 

This second solution is made by dissolving eighty grains of 
the salt in one ounce of distilled water. 

The sensitizing solution is made up as follows : 

No. 1 22 fluid drachms 

No. 2 21 fluid drachms 

Distilled water .... 4 fluid drachms 

This sensitizing solution must be made up as wanted, and 
must be kept from the light. 

How take the dry sized paper and fold its edges over a sheet 
of glass of nearly the same size, or pin it down upon a smooth 
board, so as to secure a flat surface. For an ordinary sheet of 
paper — say, 22 x 17 inches — 2^ drachms of the sensitizing solu- 
tion should be poured on the middle of the paper and quickly 
spread all over it by rubbing gently with a pad of cotton wool.' 
This should be done in a weak white light, as (the solution be- 
ing of a . yellow color) it is otherwise difficult to see if 
the paper is properly coated. 

How hang up the sheet by its corners, and allow it to 
become just surface-dry. This ought to take not less than 
five, nor more than ten minutes. Lastly, thoroughly dry the 
paper by means of a clear fire or gas-stove. 

Paper so prepared will keep good for months if it be kept 
perfectly dry. This can only be insured by keeping the paper 
(rolled up, with surface side out) in a tin tube, which also con- 
tains calcium chloride wrapped up in a little cotton-wool and 
muslin. The latter chemical absorbs all the moisture from the 
air in the tube. 

Printing is done in a frame, in the ordinary way, but it is 
best to lay a piece of sheet ind a-ruhber at the back of the 
paper, in order to prevent the access of moisture. Almost the 
only difficulty of the platinotype })rocess (hot or cold bath) is 


THE PLATINOTYPE PEINTING PROCESS, ETC. 275 

to tell when the printing is complete. As a rule, it may be 
said that all hut the faintest details should he visible in the 
faint greenish-brown image (which consists of ferrous oxalate, 
be it remembered) which is seen when one flap of the printing 
frame is turned back, and the side of the paper in contact 
with the negative examined. 

It now remains to convert this weak ^4ron” image into a 
vigorous image in “ platinum black.” 

Make a saturated solution by dissolving 16 ounces of neu- 
tral potash oxalate in 51 ounces of hot distilled water. Place 
this in an enamelled iron dish, and heat it to a temperature of 
150 deg. F., as indicated by a thermometer immersed in the 
liquid. 

How float the exposed prints one at a time for six seconds 
each upon this hot solution. Instantly the picture appears; 
and, if all has been well and the negative is a good one, we 
obtain an exquisite engraving-like picture, in which the grada- 
tions will (in the finished print) range from soft velvety blacks 
to pure whites. 

The chemical change or reaction which takes place is a most 
beautiful one, and may be expressed as follows : 

ePeC^O^ + 3K2PtCl4 = 3Pt ■+ FeoClg 

Ferrous Oxalate and Chloro-platinite produce Platinum and Ferric 
of Potassium Chloride 

+ 2Feo(C204)3 + 6K Cl 

aiid Ferric Oxalate and Potassium 

Chloride 

The moment the ferrous oxalate touches the hot potash 
oxalate it is dissolved, and it then attacks the potassium salt, 
decomposing it and producing metallic platinum, which is de- 
ported on the paper and forms the new picture. 

It will be seen that a quantity of iron salts also remains in 
the paper. These discolor the paper, and they must be re- 
moved by soaking the developed prints in two nr three changes 
of dilute hydrochloric acid (one ounce of the acid to sixty of 
water). 

Finally the prints are washed for half an hour in running 
water, and are then dried between blotting-paper. 


276 


THE CHEMISTRY OF PHOTOGRAPHY. 


Over or under-exposure can be corrected to some extent bj 
the use of a cooler (100 deg. Fahr.) or hotter (200 deg. Fahr.) 
bath. 

By the addition of a few drops of a saturated solution of 
mercury bichloride to the developing bath, prints of a sepia 
tone can be obtained. 

The Cold-Bath Pla^hnotype Process. 

No. I. 

In this form of platinotype, the paper is Coated with a solu- 
tion containing 120 grains of ferric oxalate and one grain of 
mercury bichloride to the ounce of water. It is thoroughly 
dried, exposed to light beneath a negative, and then floated 
on a cold solution containing fifty grains of potash oxalate and 
ten grains of chloro-platinite of potassium to each ounce of 
water. The paper should at once be lifted up from the cold 
solution and laid face upwards on a glass plate. Development 
proceeds slowly, and can be stopped when desired. Or the 
cold solution may be applied to the paper with a brush if 
desired. The mercuric salt acts by increasing the reducing 
power of the ferrous oxalate. The prints require clearing with 
acid, and then washing, as in the hot process. 

No. II. 

At the Camera Club Conference in March, 1892, Mr. Willis 
announced what appears to be the crowning improvement of 
the platinotype process. By a certain modification of the 
ordinary hot-bath method the developer can be used cold^ i.e.^ at 
ordinary temperatures. The image ought to be printed-out 
rather more than when the hot-bath is employed. No details 
were given as the patent was not completed, but the new 
paper has since been placed upon the market and has given 
the greatest satisfaction. By mixing the developer (ordinary 
potash oxalate solution) wdtii glycerine, and applying it with a 
brush, the process is so far under control that great varia- 
tions can be made in the results, and very artistic effects 
produced. 


THE PLATINOTTPE PRINTING PROCESS, ETC. 277 

PiZZIGHELLI, OR PrINTING-OUT PlATINOTTPE. 

The hot process might be called the ‘‘ platinum in the paper ” 
method, as distinguished from the cold process, h^o. I., in which 
we have the platinum in the bath.” But in the method devised 
by the Austrian experimenter, Pizzighelli, we do away with 
the bath altogether, and put all the substances employed, 
developer and all, upon the paper. 

Make up the following solutions : 


a Chloro-platinite of potassium 60 grains 

Distilled water 1 ounce 

h Sodium oxalate 15 grains 

Sodium-ferric oxalate 3 drachms 

Chlorate of potash 1 grain 

Distilled water 1 ounce 


To sensitize a sheet of paper (22 x 17 inches) mix two drachms 
of a with two drachms of and apply to the (previously well- 
sized) paper as described above. 

The prepared paper is printed right out in the printing- 
frame, exactly like silver paper, and to just the depth required. 
It is then cleared with acid (1 to SO) and washed as before. It 
will be found advantageous to slightly damp the paper just 
before using, either by breathing upon it, or by passing it over 
a pan-full of hot water. The chemical changes which take 
place are practically the same as those given under the hot- 
bath process ; the moisture which is necessary to develoj^ment 
is obtained by the paper absorbing it from the air. 

Tlie advantages of this ‘‘ printing-out ” method are fewer 
spoilt prints ; and the power of inserting clouds with greater 
ease. Its disadvantages, the fact that the blacks are not nearly 
so vigorous, and that it takes much longer to print (about 
twice as long as silver paper) ; while for the hot or cold-bath 
processes the time required for printing is less than half that 
needed for silver paper. 

Mr. Willis’ patents are worked in England by the Platino- 
type Co., 29 Southampton Row, High Holborn, London"^ ; 
and his American representatives are, we believe, Willis & 


* They issue pamphlet of instructions, which is w^^ll worth writing for. 


278 


THE CHEMISTKY OF PHOTOGRAPHY. 


demerits, of Philadelphia ; but all the materials we have 
named can be obtained through the Scovill & Adams Co., 
423 Broome Street, New York City. The preparation of 
the paper for the hot process is the easiest thing possible, and 
we recommend all who desire the very best results possible to 
sensitize their own paper. The Pizziglielli paper is made, we 
believe, only in Vienna, but it can be obtained to order 
through any dealer. 

To show the rate at which the platinotype process is spread- 
ing in England, we may say that at the exhibition held in Pall 
Mall in November, 1889, out of 639 frames exhibited no fewer 
than 205 were occupied by platinotypes. Of these 183 were 
by the hot bath ; 3 by the cold bath ; and 19 by the Pizzighelli 
process. 

Platinum Toning. 

This article would scarcely be complete without some refer- 
ence to a process of toning silver prints with platinum, intro- 
duced in 1889 by Mr. Lyonel Clark. Mr. Valentine Blanchard 
had previously sold prepared paper and solutions for the same 
or a very similar process, but he did not publish his method. 

All platinum printing processes at present known are substi- 
tution proGe^^es. A “provisional” image is formed in some 
other metal, and then — by a chemical change — platinum is 
caused to replace the metal. In Willis’ platinotype the pro- 
visional image is in iron ; hut, as Mr. Clark points out, silver 
will also answer the purpose. 

Plain or matt surface paper must be employed, because 
albumen prevents the free replacement of the silver by the 
platinum. Such matt-surface paper, ready sensitized, can now 
be bought of most dealers ; or it can be prepared in the follow- 
ing way. Make up these solutions : 

A — Salting Solution. 


Gelatine 90 grains 

Ammonium chloride 60 grains 

Carbonate of soda (recrystallized) 120 grains 

Citric acid (crystals) 30 grains 

Distilled water .... 10 ounces 


THE PLATINOTYPE PKINTING PROCESS, ETC. 279 

On tins solution float any good, strong white paper, and pin 
the sheets up till dry. 

B — Sensitizing Solution. 


Silver nitrate 90 grains 

Distilled water 1 ounce 


This solution must be kept in a bottle, round wh^ch two or 
three layers of brown paper have been pasted to protect it from 
the light. 

It is now only necessary to float the salted paper upon solu- 
tion B (of which, ten or twenty ounces may be made up) ; or, 
if only a small quantity of paper is to be sensitized, the solution 
may be applied with a brush or a glass rod. But the paper 
once sensitized will not keep for more than two or three days. 

Prints are to be made on this matt-surface paper in the usual 
way ; and they should be printed rather dark, for the subse- 
quent toning with platinum will somewhat reduce them. 

The platinum toning-bath is made up as follows : 


Chloro-platinite of potassium 30 grains ^ 

Water 30 ounces 

Nitric acid 10 drops 


The silver prints must be immersed in this bath (if only a 
few at a time are done, they can be floated face down on a 
little of the platinum solution poured into a levelled dish), and 
their reddish tint changes first to brown and then to black ; 
only two or three prints should be in the bath at the same 


time. The chemical change which takes place 
sented by the following equation : 

may 

be repre- 

2Ag -r K^PtCl^ = Pt 

4 - 

2AgCl 

Silver with Chloro-platinite produces Platinum 

and 

Silver 

of Potassium 


Chloride 


4 - 

2KC1 


and 

Potassium 

Chloride. 


When the desired tone has been attained, the prints must be 
well rinsed in water, to which a little ammonia has been added. 
They are then placed in a fixing-bath (hypo, four ounces, to 
twenty of water) for twenty minutes ; and are finally well 
washed for several Inmrs in plain water. 


2S0 


THE CHEMISTEY OF PHOTOGEAPHY. 


Compared with platinotypes proper, these platiimm-toned” 
prints can hardly claim equal probability of permanency ; but 
they are more permanent than the ordinary gold toned’’ silver 
prints, over which they possess further advantages in their fine 
black tones and matt-surface. 

In conclusion, platinum printing processes are no longer “in 
the future ” ; they are firmly established, and are gaining 
ground every day. The treacherous silver prints on albumen- 
ized paper — despised by every artist — will soon become things 
of the past, and a great reproach will be wiped away from 
photography. 

APPENDIX. 

Liteeatuee of Platinum Peinting Peocesses. 

JoUENAL OF THE PhOTOGEAPHIC SoCIETY OF GeEAT BeITAIN I 

W. Willis, Jr. — Notes on the Platinotype Process, N. S., 
Yol. III. (for 1878), p. 32. 

jSj)iller, J. — The Fading of Platinotype Prints, N. S., 
Yolume III., p. 74. 

Ahiey, Cavt. — On Platinotype Deposits, N. S., Yol. XII. 
(for 1888), p. 165. 

Platinotype. — By Cajpt. Pizzighelli and Baron A. Huhl / 
translated from the German by J. F. Iselin and edited by 
Capt. Abney; published by Harrison & Sons, 59 Pall 
Hall, London; price, two shillings. 

Peoceedings of the (London) Cameea Club. 

II . II. OPitrrell. — Sulphuration of Platinotype Prints, 
Yol. I. (for 1887), p. 41. 

W. Willis. — A Decent Improvement in Platinotype (Cold 
Development), Yol. II.. (for 1888), p. 47. 

IF. Willis. — Platinotype (Hot) Printing Process, Yol. II., 
p. 99. 

IF. Willis. — Platinotype (Cold) Printing Process; Yol. II., 
p. 103. 

Blanchard, Valentine. — Platinum Toning Process, Yol. IL, 

p. 128 . 


THE PLATINOTYPE PEINTING PROCESS, ETC. 


281 


Cembrano, Stroh, Dresser, etc . — Opinions on the Pizziglielli 
(Printing-out) Platinotype Process, Yol. II., pp. 136, 147. 

Cembrano, F. de P. — Printing-out Platinotype; and a 
Comparison of Platinotype Processes, Yol. II., p. 153. 

Willis^ W . — A Lesson on the Cold-Bath Process, Yol. II., 
p. 170. 

Clarli^ Lyonel . — A I7ew Platinum Toning Process, Yol. II., 
p. 185. 

Willis, W . — Pecent Improvements in Platinotype, Yol. YI., 
pp. 53, 119. 

British Journal of Photography. 

Pizzighelli, Capt. G . — The Direct Production of Platino- 
types in the Printing Frame Without Development, 
Yol. XXXY. (for 1888), pp. 213, 230. 

Cunningham, II. II . — The Xew Platinotype Cold Develop- 
ment Process, Yol. XXXY., p. 552. 

Bedding, T . — The Xew (Pizziglielli) Platinum Process, 
Yol. XXXY., p. 566. 

Fox, F. P . — Platinotype Printing, Yol. XXXYI. (for 
1889), p. 333. 

Beach, F. G . — The Pizzighelli Platinotype Paper, Yol, 
XXXYI., p. 538. 

Beetham, W. C . — Platinotype Printing, p. 21 (for 1890). 

Pike, John . — The Platinotype Process, p. 759 (for 1890). 

The Photographic Times. 

Translations, reprints and contributed articles. 



CHAPTER XXYIIL 


REDUCING PROCESSES AND THEIR CHEMISTRY. 

Meaning of the Term “ ReductionP — As used in photog- 
raphy, the word “reduction” has three distinct meanings. It 
may mean reduction in size, or reduction to the metallic state, 
or reduction in density. It is in the latter sense that the term 
is used here. In chemistry the word reduction is used in a 
totally different sense, and is taken as the equivalent of “ de- 
oxidation,” or the removing of oxygen from a compound. To 
avoid all confusion, some writers on photography prefer to 
substitute “ weakening ” for “ reducing.” For reducing agents 
weaken the image and render it less dense. 

Necessity for Reduction . — During the development of a 
negative in the dim light of the photographer’s dark-room it 
is very easy to make the mistake of developing the negative 
too much. After fixing, the negative then presents a black 
and nearly opaque appearance. It prints badly, and so slowly 
that days of exposure to sunlight sometimes fail to produce 
the desired effect. 

The same thing may happen in the production of positives 
upon glass (transparencies or lantern-slides) ; also in the pro- 
duction of developed prints (bromides or platinotypes), or 
even in the case of ordinary silver prints. 

The remedy — partial or complete — in all these cases is 
reduction. It may be that the negative, etc., is too dense in 
certain parts only ; we must then resort to local reduction. 

Reduction Easier than Intensification. — It is generally 
acknowledged that the most difficult point in development is 
to know exactly when to stop. On the whole, it is better to 
over-develop rather than to stop development too soon. In 
the latter case, not only may density be wanting, but all the 
detail may not have been got out. It is found, too, that better 


KEDUCmG PROCESSES AND THEIR CHEMISTRY. 


283 


results are obtained from a negative which has been reduced 
than from one that has been intensified. 

What Hardwick Meant hy Reduction P — In all the nine 
editions of Hardwich’s “Manual of Photographic Chemistry” 
(1855-83) he uses the term “ reduction ” in its chemical sense. 
As he points out clearly enough, a develojoing agent is — 
chemically speaking — a “ reducing ” agent ; that is, it causes a 
separation of some metal — almost invariably silver — from the 
non-metallic element or elements with which it may be com- 
bined ; and this “ reduced ” silver then composes the photo- 
graphic image or picture. 

Thus Hardwich’s reduction is not our reduction. 'Nor have 
we been able to find in his once-popular and largely-read book 
any term which is equivalent to reduction when it means the 
lessening of the density of a negative, etc. 

We can only explain this by remembering that the need for 
such a method was not anything like so great wfith collodion as 
with gelatine. The worker with collodion huilt up his picture 
by the aid of plenty of yellow light, and by the addition of 
silver to the developer. He could tell exactly when to stop. 
Moreover, the collodion negative was, as a rule, developed at 
the time and at the place when and where it was taken. And 
if the negative did not turn out well, it was cleaned ofi the 
glass and another exposure made. The wet-collodion worker 
knew what he was taking home. 

The text-books of Hunt, Lake-Price, etc., contemporary 
with the earlier editions of Hardwich, agree with that author 
in neglecting to treat of reduction. 

ReductioW^ of Residues. — Another example of the use of 
the term “ reduction,” in its chemical sense, is the way in 
which it is universally applied to the converting of photog- 
raphers’ residues (which contain gold, silver, etc.) to the 
metallic state. Perhaps some future photographic congress 
will issue a revised nomenclature of words employed techni- 
cally in our art, so that each term shall have a fixed and definite 
meaning. 

Reduction Processes Used in Collodion Times. — Still, the 
photographic periodicals of the wet-collodion times (1853-79) 


2S4: THE CHEMISTRY OF PHOTOGRAPHY. 

sliow US that reduction was not uncommonly practised, at all 
events during the latter half of that epoch. One favorite re 
ducer appears to have been a solution of iodine with iodide of 
potassium and cyanide of potassium. The iodine attacked the 
image, converting some of its silver into silver iodide ; and 
this was then dissolved away by the cyanide. 

Mr. K. Kennett gave the following formula in 1879 
‘‘Take cyanide of potassium, 10 grains; water, 1 ounce; to 
this add crystals of iodine as long as any will dissolve. With 
a camel-hair brush paint this over the parts to be reduced. 
Then wash well and dry.” 

Mr. Stillman states f that the same reducer is good for gela- 
tine films. He writes : “ I put enough of the usual solution 
of iodine with iodide of potassium, with the quantity of water 
required to flood the plate copiously, to give it a good port- 
wine color, and then add a concentrated solution of cyanide 
of potassium until the color disappears and is replaced by 
opalescence.” The plate to be reduced is soaked in water and 
then placed in the above solution till reduced. The dish must 
be rocked frequently. Mr. Stillman adds : “ I have reduced a 
plate over intensified by carelessness in the mercury solution 
until it had become perfectly orange and imprintable, without 
stain or marking, or losing the most delicate detail. But the 
plate must be carefully washed between all the operations, and 
leave no trace of the hypo in the film.” 

Ferric Chloride as a Reducer . — When a solution of ferric 
chloride or “ chloride of iron ” is poured upon a negative, it 
combines with the silver of the image to form silver chloride, 
which, being white and translucent, lowers the density con- 
siderably. The action which takes place may be represented 
by a chemical equation : 

Ags + FegClg = 2AgCl 4- 2FeCl3 

Silver Ferric Chloride produce Silver Chloride and Ferrous Chloride. 

The silver chloride must be removed by placing the (washed) 
negative in an ordinary fixing bath of hypo, and the negative 
is then to be finally washed and dried. 


Photo News Year Book,” p. G7. 
t“ British Journal Almanac,” 1883, p. 142. 


REDUCING PROCESSES AND THEIR CHEMISTRY. 285 

The difficulty in this method is to know exactly when to re- 
move the negative from the ferric-chloride bath. The best 
plan is to use a glass dish, and watch the reduction carefully 
by the aid of light reflected upward through the bottom of 
the dish from a looking-glass placed at an angle beneath. 

The strength of the solution of ferric chloride is not very 
material ; it should be of the color of sherry wine, which may 
be produced by adding four grains of the solid chloride to 
every ounce of water. Two or three drops of hydrochloric 
acid to each ounce of water is also an improvement. About 
five minutes in this solution will be sufficient to eflect a mod- 
erate reduction, followed by ten minutes in ordinary hypo 
solution. If the reduction is not found sufficient, the negative 
must be well washed and the process repeated. The method 
is also useful for clearing yellow stains, etc., from negatives 
and for removing surface fog. 

Decolorizing Negatives Reduced Toy Ferric Chloride. — The 
ferric chloride reducer too frequently leaves a yellow stain be- 
hind. M. E. Audra^' removes this in the following way : To 
a 10 per cent, solution of sulphite of soda in water add sul- 
phuric acid, drop by droj), until there is a distinct smell of sul- 
phurous acid. Immerse the stained negative in this solution, 
by which it will speedily be cleared. This is stated to be also 
a good clearing agent for pyro stains. 

Ferric Sidphate {jpersuljpliate of iron) as a Reducer. — A 
solution of ferric sulphate in water, of the strength of three 
grains to the ounce, acts as a powerful reducer. Its use for 
this purpose was described by Professor Yogel in 1886. The 
sulphate dissolves but slowly in water, so it should be stirred 
well with a glass rod and allowed to stand for half an hour 
before using. 

The chemical reaction is : 

Ag2 + Fe2(S04)3 = AgsSO^ + 2FeS04 

Silver and Ferric produce Silver Sulphate and Ferrous Sulphate., 

The silver sulphate dissolves slowly in the water as it is pro- 
duced ; but the negative should afterwards be soaked for ten 


*“ British Journal Almanac,” for 1884, p. 48. 


286 THE CHEMISTEY OF PHOTOGEAPHY. 

minutes in a weak lijpo fixing bath, which rapidly dissolves 
the silver sulphate. Afterwards wash and dry. 

Cop])eT Chloride as a Reducer, — Chloride of copper — or 
cupric chloride, as it is more correctly called — acts in exactly 
the same way as ferric chloride. It combines with the silver 
of the negative to form silver chloride : 

Agg ’ + 2CuCl2 = 2AgCi + CugClg 

Silver and Cupric Chloride produce Silver Chloride and Cuprous Chloride. 

The negative must then be immersed in the ordinary hypo 
fixing bath to remove the silver chloride. After washing, it 
should be soaked in an acid and alum clearing bath, which 
will remove the cuprous chloride. It must then be finally 
washed and dried. 

The reducing solution may be made up of three grains of 
solid copper chloride to every ounce of water. It should be 
of a pale blue color. 

Where the copper chloride is not at hand, it can be made by 
mixing 4 grains of copper sulphate and 6 grains of sodium 
chloride to 1 ounce of water. 

CuSO^ + 2NaCl = CuClg + 

Copper Sulphate and Sodium Chloride produce Cupric Chloride and 

NagS 04 

Sodium Sulphate. 

The mixture can be used for reducing just as it is, as the 
presence of the sodium sulphate makes no difierence. 

Spiller'^s Reducer. — Mr. Spiller recommends the following 
form of the copper chloride reducer. Make up two stock 
solutions : 

A. — Alum 4 ounces 

Copper sulphate (bluestone) 4 ounces 

Common salt 8 ounces 

Water 1 quart 

B. — A saturated and filtered solution of common salt. 

Mix these two solutions in equal parts — say, 3 ounces of 
each — and immerse the negative in the mixture. If the nega- 
tive is very dense, 4 or 5 ounces of B may be used to 3 of A. 
When reduction has been effected, soak for ten minutes in B 
alone ; then wash with j^lain water and dry. 


REDUCING PROCESSES AND THEIR CHEMISTRY. 287 

The chemical action may be expressed in a single equa- 
tion: 

2 CUS 04 + 4NaCl + Aga = 

Copper Sulphate and Sodium Chloride and Silver produce 
2AgCl + 2Na3S04 + Cu^Cl^ 

Silver Chloride and Sodium Sulphate and Cuprous Chloride. 

The cuprous chloride and the silver chloride are both dis- 
solved by the (B) solution of common salt. 

Copper Sulphate as a Reducer. — Dissolve half an ounce of 
copper sulphate in a pint of water. Add ammonia, drop 
by drop, until the precipitate which is hrst formed just 
disappears. 

Make a solution of 1 ounce of hyposulphite of soda in 10 
ounces of water. Soak in this the negative to be reduced. 
Add a few drops of tlie copper sulphate solution and soak 
well; add more copper as required. Afterwards wash well 
and dry. 

Aga + Cu(NH 3 ) 2 (S 04)2 + 2 H 2 O = 

Silver and Ammonia-Sulphate of Copper and Water produce 
x\g 2 S 04 + CuHgOo + (NH4)2S0^ 

Silver Sulphate and Copper Hydrate and Ammonium Sulphate. 

The chemical action may be represented by the above equa- 
tion. Silver sulphate is formed, and then this is dissolved 
away by the hypo solution. 

Ozone BleaclC'^ and other Ilypochloritm as Reducers , — 
The substance known as ‘Mlolmes’ Ozone Bleach” was at one 
time much used (its price was only eight-pence per quart 
bottle) for laundry work and as a disinfectant. Chemically it 
is sodium hypochlorite. It was recommended by Mr. W. E. 
Debenham as a reducer in 1881 ; and he gave the following 
formula in 1882 : 


Ozone bleach. bounce 

Chrome alum 10 grains 

Water 5 ounces 


The negative to be reduced is soaked in the above solution 
for a few minutes, during which time part of the image is 


288 


THE CHEMISTRY OF PHOTOGRAPHY. 


converted into silver chloride according to the following 
equation : 

Agg + 2NaOCl + H 3 O = 2AgCl + 

Silver and Sodium and Water produce Silver and 

Hypochlorite Chloride 

2NaHO + O 
Sodium Hydrate and Oxygen 

The negative is then dipped into hypo, which removes the 
silver chloride, and the image is thus reduced.” 

The object of the chrome alum is to harden the gelatine. 
If it is dispensed with, the hypochlorite attacks both the 
gelatine and the silver, and the surface of the film is converted 
into a slimy layer which should be rubbed off with a pad of 
cotton- wool. Some have preferred to use this reducer in this 
fashion ; but the addition of the chrome alum brings the 
reduction more under control. 

Other Hypochlorites act in a like manner to “ ozone bleach” ; 
and since the latter appears to be not now readily obtainable, 
they may be used instead of it with identical results. Thus 
we have Robinson’s method * of mixing hypochlorite of sodium 
(two parts of the commercial solution) with one part of a satu- 
rated solution of alum. Filter; and then bottle for use. 
Robinson gives the following equation : 


2 A 1 K(S 04)2 

Alum 

-f 6NaC10 

and Sodium Hypochlorite 

-h 6 H 2 O 

and Water 

= 

p AlgHgOg 

- 1 - 

SNagSO^ 

produce 

Aluminium Hydrate 

and 

Sodium Sulphate 

4- 

K^SO, 


6HC10 

and 

Potassium Sulphate 

and 

Hypochlorous Acid 


"When the solution is poured upon a negative, the hypo- 
chlorous acid combines with the silver of the image to form 
silver chloride, which can afterwards be removed by a bath of 
ordinary hypo. 

Ags + 2HC10 2AgCl 

Silver and Hypochlorous Acid produce Silver Chloride 

+ HgO + O 

and Water and Oxygen 


* Photographic News, 1887, p. 499. 


EEDTTCIlSrQ- PROCESSES AND THEIR CHEMISTRY. 


289 


Of the bottled solution as described, one part should be 
mixed with three parts of water for use. 

The negative should be removed from the reducing solution 
a little h^ore the reduction required has been obtained ; since 
the action will continue for some time while the reducer is in 
course of being washed out, and also because the removal of 
the silver chloride by the hypo causes a slight further loss of 
density. 

Reduction with '•^Bleaching Powder P — Ordinary bleaching 
powder is calcium hypochlorite. It may be dissolved in water 
in the proportion of 20 grains to the ounce, and filtered ; and 
will then reduce any negative which may be soaked in it. 

The action is as follows : 

CaClgOg + Aga = 2AgCl + 

Calcium Hypochlorite and Silver produce Silver Chloride and 
CaO 4 - O 

Calcium Oxide and Oxygen 

The negative must then be well rinsed,^ soaked in a bath of 
hypo (to remove the silver chloride), washed and dried. 

Great care must be taken in using this method, or the gela- 
tine will be so softened that the film will come away. 

This reducer was described in the Photo News Year-Book ” 
for 1886, p. 37, by Mr. P. W. Pobinson. He points out that 
the negative must be previously thoroughly dried^ and that the 
reduction must be stopped as soon as the film begins to feel 
slimy. Local reduction can be effected by gentle rubbing wdth 
the linger on the part desired. 

Eau de Jamlle as a Reducer. liquid known as eau 
de javelle” is sold commercially, and a good reducer may thus 
be made : 

Eau de javelle 1 ounce 

Chrome alum 25 grains 

Water 10 ounces 

Dissolve the alum in the water, and then add the eau de 
javelle. The mixture is at first green and turbid, but becomes 
a clear yellow in a few minutes. 

The negative must be soaked, first in water, and then in the 


290 


THE CHEMISTEY OF PHOTOGEAPHY. 


above solution till reduced. Finally, wash well, fix in ordinary 
hypo, wash again, and dry. 

Where eau de javelle cannot be purchased, a reducing solu> 
tion containing it can be made up as follows : 


Dry chloride of lime 2 ounces 

Carbonate of potash 4 ounces 

Water 30 ounces 


Dissolve the lime in 20 ounces of the water, and the potash 
in the remaining 10 ounces. Mix, boil, and filter. This is the 
reducing solution in which the negative is to be immersed. 
The subsequent operations are the same as those described 
above. 

Reducing with Co7idy^s Fluid. — Mr. W. C. Williams gave 
the following method in 1881 


Condy’s fluid 4 'iram 

Saturated solution of alum, 4 ounces 


Condy’s fluid is a solution of permanganate of potash ; and 
instead of buying the bottled “fluid,” the solid permanganate 
may be dissolved in water in the proportion of 2 grains per 
ounce. 

The negative is to be soaked in the mixture for ten or 
fifteen minutes, then washed, soaked in ordinary hypo solution, 
well washed, and finally dried. We find that this method 
stains the negative yellow or red, and we do not recommend 
it, although it certainly reduces the image eflectiially enough. 

Reducing with Bichroinate of Potash. — Make up the 


following solution : 

Bichromate of potash 2 parts 

Sulphuric acid 4 parts 

Water. 100 parts 


Soak the dried negative in the water for an hour, and then 
immerse in the above solution. Watch its action carefully, as 
it reduces quickly. This method was recommended by Mr. 
W. Hanson in the British Joiornal of Photography for 
February 23, 1872. 

Farmer'' s Femddcyanide Reducer. — The most “popular” 


'^British Journal of Photography y Vol. XXVIII, p. 29. 


KEDUCING PROCESSES AND THEIR CHEMISTRY. 291 

reducer of the present day is probably that first recommended 
in 1883 by Mr. Howard Farmer (instructor in photography at 
the Polytechnic Institution, London), and described by him in 
detail in the ‘‘ Photo Hews Year-Book” for 1881. Make up 
the following solutions : 


A. — Ferndcyanide of potassium 1 ounce 

Water 1 pi»t 

B. — Hyposulphite of soda 1 ounce 

Water 1 pint 


Soak the negative in the hypo solution for a few minutes, 
and then pour off the hypo into a glass vessel. Add to the 
hypo a few drops of the ferridcyanide solution (enough to 
make it sherry colored), and pour the mixture over the nega- 
tive. Grradual reduction will take place. When the action 
stops add a few more drops of the ferridcyanide if the reduc- 
tion is not sufficient. Then remove the negative, wash well, 
and dry. The operation is best performed in a weak light. 
The mixed solutions do not keep. 

Ferridcyanide {oy f err icy anide) of potassium is commonly 
called red prussiate of potash. It combines with the silver of 
the image to form silver ferrocyanide, and then this is dis- 
solved away by the hypo, the negative tlius becoming thinner 
and thinner. The first action is expressed by the equation : 

2Agg + 2KeFe,(CN)i ^ = Ag,Fe(CN)e 

Silver and Potassium Ferridcyanide produce Silver Ferrocyanide 
+ 3K4Fe(CN)6 

and Potassium Ferrocyanide. 

The hypo then acts as follows : 

Ag4Fe(CN)e + 4NagS203 = 

Silver Ferrocyanide and Sodium Hyposulphite produce 
If tAgNaSgO^ -I- Na4Fe(CN)6 

Silver Sodium Hyposulphite and Sodium Iron Ferricyanide, 

As the two chemical actions take place simultaneously, the 
reduction can be seen as it progresses and arrested at any mo- 
ment. This is one of the best points of the process. The 
mixture, however, does not keep well, so that it cannot be pre- 
pared and bottled for use, but must be made as required, 


292 THE CHEMISTRY OF PHOTOGRAPHY. 

As stated in another paragraph, this method can also be 
used for reducing silver prints. 

Belitzki^s Ferric Oxalate Feducer . — The following reducer 
is due to Herr L. Belitzki, of Hordhausen. 

Make up the following solution ; 


A. — Ferric chloride 10 grains 

Potash oxalate 3^ ounce 

Water I 34 ounce 


Chemical combination will take place and ferric oxalate will 
be formed, as shown by the equation : 

FegClg + 3K2C2O4 r= Fe2(C204)g + 

Ferric Chloride and A-^otash Oxalate produce Ferric Oxalate and 

6KC1 

Potassium Chloride. 

The ferric oxalate so formed is ready to act as our reducer ; 
but we must also have some substance present which will dis- 
solve or wash off the silver oxalate which will be formed. 
Such a substance is found in sodium hyposulphite. Make up, 
therefore, a second solution as follows : 

B. — Hypo. 

Water 

Mix this with solution A, as given above. 

When the negative to be reduced* is placed in the mixed 
solutions A and B, the ferric oxalate at once combines with 
the silver of the image to form silver oxalate. 

Agg + Feg(C 204)3 = Ag 2 Cg 04 + 

Silver and Ferric Oxalate produce Silver Oxalate and 

2 FeCg 04 
Iron Oxalate. 

Then the hypo combines with the silver oxalate to form a 
soluble salt, which is washed olf by the water. % 

+ 2 NagS 203 = 

Silver Oxalate ayid Sodium Hyposulphite produce 

2 AgNaSo 03 + Na 2 Cg 04 

Silver Sodium Hyposulphite ana Sodium Oxalate. 

The negative must be carefully watched while in the reduc- 


34 ounce 
2 ounces 


* If the neprative has been dried it should be well washed before reducing. 


EEDUCma PROCESSES AND THEIR CHEMISTRY. 


293 


ing solutioD, so tliat it may not be made too weak. The action 
is, however, very steady and gradual. Occasionally the plate 
may be lifted out and examined by transmitted light. 

The negative is finally well washed and dried. This method 
of reduction has been approved by that high authority, Dr. 
Eder, and is sometimes known as “EdeEs” reducer. 

A B^ducer Contained in Used Ferrous Oxalate. — Instead 
of the mixtui-e of ferric chloride and potash oxalate men- 
tioned above, the green crystals deposited in all old solutions 
of ferrous oxalate developer may be used. This form of 
Belitzki’s reducer is described in the Photogra]pMc News for 
25th of January, 1884. The green crystals consist of the 
double oxalate of iron and potash. The reducing solution 
may then be made up as follows : 

Green crystals ounce 

Hypo 1 ounce 

Water. 5 ounces 

Use as before ; the chemical action is the same. 

We see now the reason why negatives developed with old 
ferrous oxalate are usually thin. The ferric oxalate present 
acts as a reducer. 

Belitzki^s Durable Reducer. — As an improvement on the 
method just described, Herr Belitzki gave the following in 
1890. 

Dissolve in the order given : 


Water 200 parts 

Ferric-potassic oxalate 10 parts 

Sodium sulphite (neutral) 8 parts 

Oxalic acid 2^^ parts 


Of sodium hyposulphite solution (1 to 4) 50 parts 

This solution keeps well if filtered and placed in opaque 
stoppered bottles. It can be used immediately after fixing. 
It will be noticed that the proportions of this solution are 
given in “parts.” This means by loeight. 

These methods of reducing with ferric oxalate are now 
usually assigned to Belitzki and Eder. But it is practically 
the same thing as the method for “clearing” negatives de- 
scribed by W. Willis before the Photographic Society of 


294 


THE CHEMISTRY OF PHOTOGRAPHY. 


Great Britain in 1882, and commented on in the British 
Journal of Photography for the 7th of July, 1882. And 
even before Willis the method had been proposed by Monck- 
hoven. 

Lainerh Reducing Bath with Sulphurous Acid, — In the 
Photographische Correspondem for 1890, A. Lainer recom, 
mends the following reducer to be used conjointly with his 
acid fixing bath : 

Hypo .... 2 ounces 

Sodium sulphite 4 ounces 

Hydrochloric acid 34 ounce 

Water 10 ounces 

The negative must be left in this solution until reduction is 
effected, and the time required for this may be twelve hours. 
As sulphurous acid is given off, the vessel containing the solu- 
tion must be provided with an air-tight cover, or it may be 
covered over and left out of doors. 

Valuable Cl earing -a^id- Reducing Agents, — For many years 
we have recommendod that all negatives should be passed 
through the following bath, but they must first have been 


thoroughly well washed : 

Ferrous sulphate 34 ounce 

Hydrochloric acid drachms 

Saturated solution of alum 4 ounces 

Water 2 ounces 


This clears in a marvellous way the yellow stain caused by 
development with pyro; and it also slightly reduces. The 
solution should be kept in motion (by rocking the dish) while 
upon the negative. Ten minutes in this bath will generally 
‘‘clear” the negative, which must then be rinsed and washed 
in running water for half an hour. 

Another useful “clearer and reducer” is a saturated solution 
of alum (say, 10 fiuid ounces), to which half an ounce (or less) 
of strong sulpliuric acid has been added. From two to five 
minutes in this solution will usually be sufficient. 

BurtoJs Reducer. — Harden the gelatine film thoroughly by 
soaking for an hour in a saturated solution of chrome alum. 
Wash for ten minutes and dry. Now squeegee on the bach of 


295 


KEDgCING PROCESSES AND THEIR CHEMISTRY. 

the glass a piece of sensitized carbon tissue. When dry ex- 
pose to light (film facing the light) in a printing frame 
as usual. Remove, and develop the carbon print by soaking 
in warm water. The second coating (on the hack of the nega- 
tive) will then correct any violent effects in lighting which 
may be produced by the original negative. This method is 
best suited for clialky black-and-white negatives, and requires 
a skilful operator. It is not a reducer in the sense of lessening 
or changing the substance of the image ; but it reduces ” the 
violent contrasts caused by under-exposure or faulty lighting. 

Local Reduction 

It is frequently the case that only a small part or parts of a 
negative require reduction. To obtain local reduction two 
plans are open to us. We may do it either chemically or 
mechanically. 

1. Cliemical Methods for Local Reduction . — In this first 
way the negative should be well soaked in water until the film 
is quite soft. The surface moisture is then taken off with blot- 
ting-paper, and any of the chemical reducing agents which we 
have described may be applied to the parts requiring reduction 
by means of a finely pointed camel-hair brush. As an illus- 
tration of this method. Professor Vogel relates that a short 
time ago he “ took a view in Torgatten, Norway, of a rocky 
cave looking out upon the sea. As was expected, the opening 
of the cave was considerably over-exposed, and was also sur- 
rounded by an ugly halo. In order to reduce this portion 
without affecting the rest, the negative was soaked in water 
till thoroughly wet, and then the portion not to be reduced was 
dried with strips of blotting paper. Holding the plate liori- 
zontally, a solution of persulphate of iron was applied to the 
portion to be reduced, while the effect was watched by the 
aid of the light reflected from a piece of looking-glass held 
under the negative. The effect was so striking that after a few 
minutes not only the halo disappeared, but the whole of the 
over-exposed part of the landscape was reduced to the required 
density. Nothing remained but to wash the plate in a thor- 
ough manner for one hour.” 


296 THE CHEMISTRY OF PHOTOGRAPHY^ 

Another method is to paint with some tough varnish all 
round the part which has to be reduced. Allow the varnish to 
dry, and then apply the reducing fluid with a brush. Kemove 
the varnish afterwards by warm methylated spirit. 

2. Mechanical Methods for Local Reduction. — {di) In the 
British Journal of Photography for 8th of September, 1882, 
Mr. W. E. Debenham speaks of “the removal to a certain 
depth of the gelatine film with the image it bears. This be- 
ing effected by the rubbing away of part of the negative with 
a line cutting powder, such as cuttle-fish bone.” The powder 
may be applied with the tip of the finger. Cigar-ash may be 
used in like manner. 

{Ij) A better method (and, in fact, the best method for local 
reduction with which we are acquainted) was described by Mr. 
W. Brooks in the British Journal for 1884, p. 633 ; and 1885, 
p. 343. 

A little strong alcohol (methylated spirit answers well, if of 
best quality and of specific gravity not higher than .825) is 
poured into a saucer ; the negative is placed on a retouching 
desk (a printing-frame with the back removed will do) and a 
sheet of white paper is placed so as to reflect the light through 
the negative, and so to enable the reduction to be stopped when 
sufficient. A piece of wash-leather or a fine linen rag is 
dipped into the alcohol and then rubbed upon the part of the 
negative which it is desired to reduce. The surface of the 
gelatine film is gradually rubbed away, and the silver which it 
contains is seen as a black stain upon the wash-leather. Dip the 
latter into the spirit every minute or two, and continue rubbing 
until the over-dense parts have been sufficiently reduced. 

It is often marvellous to see how this method will reveal the 
buried detail in opaque faces, hands, white lace collars, etc., of 
portraits ; or bring to light the tracery of church windows, re- 
duce the halos round them, etc. It is a good plan to tie a 
piece of cotton -wool inside the wash-leather, so as to make a 
little pad, which is easy to handle. To rub down fine lines a 
pointed piece of wood may be covered with wash-leather. 
White, “snowy” patches in the foregrounds of landscapes are 
readily “ rubbed down.” The work is done most easily just 


REDUCING PROCESSES AND THEIR CHEMISTRY. 


297 


after the negative has been dried. With an old negative it is 
well to soak it in water for an hour, then dry and commence 
rubbing. Do not be afraid to rub liard, but take care that no 
grit gets on the rubber or it will cause scratches. 

When the reduction seems sufficient (and be careful not to 
over-do it) rub the film all over with a clean rag and some 
spirit, which will remove any smears, and take a print to see if 
the desired result has been obtained. 

(c) The Kn^fe. A method of reduction which requires 
considerable skill in its use has of late come into great favor 
among professional retouchers. It consists in the use of a 
knife-blade ground to a very fine edge and with a rounded 
point, by which the surface of the gelatine film is carefully 
scraped away and shaved off in fact, and marvellous alterations 
effected. By this means ladies’ waists are contracted, their 
hands diminished, etc.; in fact, almost any tricks can be played 
with the negative. But to do this well requires long practice, 
combined with manual dexterity. A mezzo-tint scraper is a 
useful tool where fine lines have to be thinned. 

Removal of Stains on Negatives. — The small brown spots 
or stains seen on most negatives, which have been printed 
from without being varnished, can generally be removed by 
the aid of an alcoholic solution of cyanide of potassium of 
the strength of three grains of the cyanide to each ounce of 
strong alcohol. The negative should be soaked in this and 
gently rubbed with cotton-wool. Then soak in alcohol alone, 
and finally wash well in water and dry. 

Deduction of Proofs on Paper. 

Reduction of Over-printed Silver Prints on Albumenizeu^ 
Paper. — It is not an easy thing to learn the exact degree of 
over-printing which is necessary to furnish the perfect ” 
print, after the slight reduction which it undergoes in the sub- 
sequent operations of toning and fixing. 

Again, having learned the exact shade or depth required, it 
is not always the case that the print is removed from the 
printing-sframe at the right moment. With the professional 
printer, whose whole time is given to the looking after a large 


298 


THE CHEMISTRr OF PHOTOGEA.PHY. 


number of frames, doubtless the percentage of prints spoiled 
by over printing is very small; but with the average amateur 
it is large. Other objects direct his attention ; the power of 
the light is under-estimated, and then, when the frame is 
opened, the print is as black as my hat.” 

Several attempts have been made to discover processes by 
which such over-pi inted prints could be made passable. As a 
rule these processes result in failures. Not that they do not 
reduce the prints ; but that at the same time the tone or color 
of the prints is altered, and for the worse, while mealiness is 
frequently produced. 

But it is not unfrequently the case that it is desirable to 
save the print, even if the result be somewhat inferior to 
what we could desire. In such cases the following methods 
may be tried : 

Reduction of Prints with Hypo. — If the over-printing be 
only slight, reduction may be effected by leaving the print for 
an hour or two in a fresh solution of hyposulphite of soda. 
Given time enougli, the hypo will dissolve the finely divided 
silver of which the image is composed. The hypo bath used 
for this purpose should be fairly strong (say 6 ounces of hypo 
to 20 ounces of water), and its temperature should be about 
70 deg. Fahr. 

2. England's Method with Cyanide of Potassium. — In 
1881 Mr. William England recommended* a bath ‘‘of only 
four drops of saturated solution of cyanide of potassium to a 
pint of water” for the reduction of silver prints. 

In this extremely weak bath the prints show no signs of 
change until about an hour has elapsed. They must then be 
removed, washed well in water, and dried. Cyanide of potas- 
sium had been used for a like purpose long before 1881, but 
not with success. The new point in this method consists in 
the extremely dilute state in which the solution is employed. 
Chemically speaking, the action is simply that the cyanide 
combines with the silver to form a soluble compound. 

3. Dunmore s Method with Mercury Bichloride.^ 1890.-— 

Make up the following solution : v. 


* “Journal of the Photographic Society,” New Series, Vol. V., p. 138. 


EEDUCING PROCESSES AND THEIR CHEMISTRY. 299 


Mercury bichloride 12 grains 

Potassium bromide 12 grains 

Water 4 ounces 


In this immerse the dark prints, and watch them carefully 
until they are of the right depth, which will be in a few min- 
utes. Mr. Dunmore states that prints which he treated in 
this way ten years ago have not faded. The solution can be 
used repeatedly until its strength is exhausted. 

4. With Common Salt . — After the dark prints have been 
toned, fixed, and dried, they are placed in fresh hypo solution 
of the usual strength, to which a little methylated spirit has 
been added. The following answers well : 


Hyposulphite of soda 2 ounces 

Methylated spirit 2 ounces 

Water 10 ounces 


After soaking for ten minutes transfer the prints to a satu- 
rated solution of common salt, and after five minutes put 
them back into the hypo again. With extremely black prints 
about five drops of a saturated solution of cyanide of potas- 
sium may be added to the salt bath. 

5. With Alkaline Ferridcyanide.^ — Farmer’s reducer, 
which has met with so much favor for reducing negatives, has 
been condemned for prints ; but by the addition of an alkali 
it is, according to Mr. Sherman, capable of acting as efficiently 
upon over-printed silver prints as upon negatives. Make up 
the following stock solutions : 


A. — Ferridcyanide of potassium 1 ounce 

Water 1 pint 

B. — Carbonate of ammonia 1 pound 

Water 5 pints 

This is a saturated solution. 

C. — Hyposulphite of soda 1 ounce 

Water 10 ounces 


For use, add to C 1 drachm of B, and enough of A to 
make it a lio;ht lemon color. 

o 


* W. H. Sherman, in “ Photo Mosaics,” for 1888. 


mo 


THE CHEMISTEY OF PHOTOGEAPHY. 


Put the mixture in a white dish and immerse the prints to 
be reduced (one at a time) in it. Pemove them to a bath of 
salt-water (a handful of salt to a gallon of water) when suffi- 
ciently reduced. Then wash well and dry. This method also 
improves yellow prints and prints which have been made on 
stale paper. 

6. By using the chloride of lime toning bath, or by toning 
with platinum, over-done prints can frequently be persuaded 
to assume a respectable appearance. Each of these toning 
methods, in fact, requires a certain amount of over-printing. 

Reduction of Bromide Prints. — Bromide prints can be re- 
duced in just the same way as negatives. 

Reducing Over^ Printed Blue Prints P — 1 . Soak the 

prints in 


Potassium carbonate 100 grains 

Water 12 ounces 


They will gradually be reduced. Then rinse, wash for five 
minutes, and immerse for a few seconds in 

Acetic acid 25 minims 

\Vater 4 ounces 

This brightens up the prints. Finally wash for ten minutes 
and dry. This method was described by Messrs. J. P. and F. 
C. Beach in 1888. 

2. Dip the prints first into a five per cent, solution of am- 
monia, and then into hydrochloric acid of the same strength. 
Dilute these solutions if they act too rapidly. 

Classification of Peducing Peocesses.* 

The various reducers whose action we have now described, 
may be arranged in six classes : 

First. — A change in the color of the film or deposit, where- 
by it is made more transparent to the chemical rays. Example 
— («) Clearing of stains, etc., from film by action of mixture 
of hydrochloric acid and alum. If) Bleaching of film by 
bichloride of mercury. 


* According- to Mr. W. E Debenham. 


REDUCING PROCESSES AND THEIR CHEMISTRY. 


301 


Second . — A direct solution of a portion of the silver embed- 
ded in the gelatine film, and constituting the negative. Ex- 
ample. — {a) By ozone-bleach, and other hypochlorites. (These 
have a chemical action also.) 

Third . — A chemical change of a portion of the deposit into 
a compound, which may afterwards he dissolved in a proper 
solvent. Example. — Most reducing processes belong to this 
class, as ferric chloride, mercuric chloride, ferricyanide of 
potassium, etc. These must be followed by the application of 
hypo to remove the silver salts which are formed. 

Fourth . — A solution or loosening of the gelatine film itself. 
Example. — The action of bleaching-powder and other hypo- 
chlorites. This method is not applicable to collodion nega- 
tives. 

Fifth . — A rubbing or cutting down of the dry gelatine film, 
as by the use of alcohol applied on wash-leather, etc. 

To these may be added : 

Sixth . — Any method of working upon the glass hach of the 
negative, as Burton’s reducer, or by stretching a piece of tissue- 
paper over the back and then working upon it with lead-pencil 
or the stump, by which extreme contrasts are reduced. 

Maxims for the Beducing of B^egatives. 

1. If the negative is varnished, the varnish must be removed 
before reducing, by means of warm methylated spirit, aided 
by gentle friction with a pad of cotton-wool. 

2. The negative must be thoroughly well washed to free it 
from hypo before reduction is attempted (except in the special 
cases noted). 

3. Keep the dish in motion while the reducing solution is 
upon the negative, or the action may be unequal. 

4. For negatives which may have been fixed in an acid 
fixing-bath, the ‘^acid ferric oxalate” (durable reducer) of 
Belitzki should be used. 

5. In other cases try first the reducing solution of red prus- 
siate of potash followed by hypo (Farmer’s reducer). 

6. Be move the negative from the reducing solution while 


302 


THE CHEMISTRY OF PHOTOGRAPHY. 


yet a little too dense. It will lose slightly in washing and 
drying. 

Bibliography of Beduction. 

Papers on this Subject contained in the British Journal 
OF Photography : 

Sellers^ Coleman. — Beducing Tarnished (Collodion) Nega- 
tives with Potassium Cyanide and Alcohol (1864), p. 31. 

Lea., Carey. — On Beducing Over-Printed Proofs (1865), p. 
324. 

{From Humphrey'^ s Journal^ — What to do when a Nega- 
tive has been Intensified too much (1866), p. 610. 

England., W. — Perfecting, etc., of Negatives (186T), pp. 
24, 56. 

Lea, Carey. — Over-developed Negatives (1869), p. 204. 

LLanson, Wm. — Beducing the Intensity of Negatives (1872), 

‘ p. 93. 

Letalle, A. — Beducing the Intensity of Negatives (1873), p. 
74. (Gold Chloride and Nitric Acid.) 

{Leader^ — Beducing the Density of Gelatine Negatives 
(1879), p. 311. 

{Leader.) — Clearing the Shadows of Gelatine Negatives 
(1880), p. 542. 

Blomehard, Y . — Bemoving the Color from Gelatine Nega- 
tives (1880), p. 571. 

Cowan, A. —Beducing the Density of Gelatine Negatives 
(1881), p. 4. (Praises Debenham’s Method with 
Holmes’ Ozone Bleach.”) 

Williams, W. C. — Beducing with Condy’s Fluid (1881), 
p. 29. 

England, W. — Beducing Prints with Cyanide of Potassium 
(1881), p. 264. 

Smith, George. — Beducing Gelatine Negatives with Mer- 
cury and Ammonia (1882), p. 315. 

{Leader) — Ferric Oxalate for Clearing, etc., Negatives 
(Willis’ Method, 1882), p. 381. 

Dehenham, W. E. — Beducing Intensity by Various Means 
(1882), p. 516. 


KEDUCING PROCESSES AND THEIR CHEMISTRY. 


303 


Cotesioorth^ II. Y. E. — Reducing with Ozone Bleach (1882), 
p. 642. 

Farmer., E. H. — Gelatine Process, Reduction of Density 
(1883), p. 119. 

{Leader?) — Reducing the Density of J^egatives (1883), p. 215. 
Alfieri^ C. — Reducing with lodocyanide of Potassium (1883), 

p. 622. 

Brooks^ Wm . — Rubbing Down with Methylated Spirit 
(1884), p. 633 and (1885), p. 343. 

Brooks^ Wm . — Silver Stains Removed by Potassium Cyam 
ide in Alcohol (1885), p. 343. 

{Leader?) — Reducing by Chloride of Lime (1887), p. 402. 
Sherman., TP. II. — Reducing Prints wdth Alkaline Ferrid- 
cyanide (1888), p. 55. 

{Leader?) — Reducing Over-printed Proofs (1890), p. 721. 
Dunmore., E — Reducing Prints with Mercury Bichloride 
(1890), p. 775. 

From The Photographic Times: 

{Editorial ?) — Reducing the Intensity of iSTegatives (1887), 

P- 3. 

{Editorial ?) — Reducing Negatives with Potassio-ferric Oxa- 
late (1888), p. 361. 

Reducing (Miscellaneous, 1888), pp. 332, 502, 312, 

507, 48. 

{Miscellaneous .) — Reducing Negatives, Prints, etc. (1889), 
pp. 260, 574, 654. 

{Miscellaneous) — Reducing, etc. (1890), pp. 171, 338, 488, 
569. 

From the Photographic News : 

{Leader) — Belitzki’s Oxalate Reducer (1884), p. 49. 

Coles. W. — Altering the Density of G-elatine Negatives 
(1884), p. 388. 

MartiEs Oxalate and Hypo Reducer (1884), p. 424. 
Sjpilleds Reducer ( 1 885), p. 10. 

{Leader) — Reducing with the Yiew of Obtaining Clear 
Shadows (1885), p. 785. 


304 


THE CHEMISTRY OF PHOTOGRAPHY. 


Ashman. W. M . — Reducing and Intensifying Negatives 
(1886), p. 306. 

Vogel, Professor . — Reducing Dense Places in Gelatine 
Negatives (1886), p. 676. 

Robinson, R. W . — Reduction of Negatives (1887), p. 499. 
See also Debenliam, p. 527 and p. 542. 

Ehrmann, Dr. C . — Reducing the Density of Negatives by 
Yarious Agents (1888), p. 506. 

Beach, J. P . — Reducing Over-printed Blue Prints (1888)^ 
p. 650. 

Gosselin, Dr . — Reduction with Acid Bichromate of Potash 
(1889), p. 880. 

BelitzMs Durable Reducer (1890), p. 989. 



CHAPTEE XXIX. 


INTENSIFYING PROCESSES, AND THEIR CHEM- 
ISTRY. 

The verb “to intensify” does not belong to the early history 
of photography. The dagiierreotypists (1839-55) do not ap- 
pear to have endeavored (or at all events were not able) to 
remedy any deficiency in the depth or density presented by 
their negatives. As the developing solution left them, so they 
had to remain. If the photographer was not then satisfied 
with the picture on his silver plate, his sole remedy was to 
take another negative. 

AYhetherthis is not still the best plan to adopt is an open, 
question — probably it is. But soon after the advent of the 
calotype process in ISdl, a method of increasing the density 
or opacity of the developed negative was discovered, and this 
method was found to be still more applicable to the collodion 
process (1851) wdiich displaced both calotype and daguerreo- 
type. To such a process the name of intensification was 
given. 

The idea of intensification is not contained in Snelling’s 
“ Dictionary of the Photographic Art,” published in Xew Y^ork 
in 1854. In Sutton’s “ Dictionary of Photography” (London, 
1868), he alludes to the subject only under the head of “devel- 
opment,” but in the second edition of this book (1867) we 
find a very good definition of “ Intensifiers” : — “This term is 
used to denote those substances which, when applied to a 
negative, serve to increase the actinic opacity of the deposit 
already formed. One class of intensifiers acts by increasing 
the deposit of silver forming the image. To this class belong 
a mixture of protosulphate of iron and nitrate of silver, also 
pyrogallic acid and nitrate of silver. The latter method is 
most commonly adopted. 

“Another class .of intensifiers derives its value not from 


306 


THE CHEMISTRY OF PHOTOGRAPHY. 


forming any new deposit, but from clianging that already 
formed to a more non-actinic color. To this class belong the 
alkaline sulphides, which blacken the silver deposit ; and 
Schlippe’s salt, which turns the deposit to a very non-actinic 
scarlet color. Several other substances act after this fashion, 
but, as a rule, they are inferior to the first class.” 

Intensification of Galotypes or Talbotypes. — It will be re- 
membered that the calotype negatives (consisting of silver 
iodide on paper) were developed by means of a solution of 
gallic acid. One of the best-known text-books on this process, 

Thornthwaite’s Guide to Photography,” 1852, adds (page 84): 
‘‘ Development can be singularly accelerated by adding a few 
drops of aceto-nitrate of silver, when the image begins to 
develop itself ; and very intense blacks are obtained by this 
method.” That is, a thin image being produced by the devel- 
oper proper (the gallic acid), this image was able to attract to 
itself more silver from the mixture of acetic acid and silver 
nitrate which constituted an intensifier^ and the image was 
thus able to build itself up, and to increase in density. 

Intensification of Albumen Negatives. — The albumen pro- 
cess of Niepce de St. Yictor, 1847, consisted in forming silver 
iodide upon a glass plate coated with white of egg (albumen). 
^Vfter exposure in the camera, the image was developed by 
pouring over it a saturated solution of gallic acid. Mr. Malone* 
writes : ‘‘A negative image is the result. At this point previ- 
ous experimentalists have stopped We have gone further^ 
and find that by pouring upon the surface of the reddish- 
brown negative image during its development a strong solu- 
tion of nitrate of silver, a remarkable effect is produced. The 
brown image deepens in intensity until it becomes black.” 

Of the same albumen pi-ocess May all also wrote (Hunt, 
1851): ‘‘After development, should the image be still feeble, 
pour off the gallic acid, rinse the proof with water, and pour 
on to it equal quantities of aceto-nitrate of silver and gallic 
acid reduced one-half with water. The image will now quickly 
develop.” 

* One of the best known of the early practical workers in photography.” The quota- 
tion is from “ Hunt’s Photography,” 1851. 


INTENSIFYING PROCESSES AND THEIR CHEM1STRY\ 


307 


Intensification of Collodion Negatives. — The method of 
intensification just described was applied but seldom in the 
case of calotype and albumen negatives, but became quite a 
constant practice in the development of collodion negatives. 
The great text-book of the worker with collodion was Hard- 
wich’s Manual of Photographic Chemistry,” of which nine 
editions appeared between 1855 and 1883. In the first edition 
stress is laid on a clear understanding of the word intensity., 
which relates to the appearance of the finished photograph, 
independently of the time taken to produce it ; to the degree 
of ojpacity of the reduced silver^ and the extent to which it 
obstructs transmitted light.” 

The film on a collodion plate consisted of iodide of silver in 
collodion ; but this was exposed wet, and covered with a solution 
of nitrate of silver. The action of light produced a change 
in the silver iodide ; and development was effected by pouring 
over the plate either a solution of pyrogallic acid, or one of 
ferrous sulphate. The result of this was to decompose the 
nitrate of silver, and tlie metallic silver was then deposited 
iipon the image produced by light. 

Thus a collodion plate was developed from above • and the 
picture could often be seen to stand out distinctly in fine 
relief upon the surface of the collodion. 

But the layer of silver nitrate upon the surface of a collodion 
wet-plate was very thin (for the plates were drained after 
removal from the nitrate bath, and before exposure), and its 
silver was soon exhausted. The developed image was conse- 
quently too weak and thin. Hardwich (1855) then recom- 
mends the following intensification : In development “ the 
pyrogallic acid is to be used alone, until the image has reached 
its maximum of intensity, which it will usually do in a minute 
or so, according to the temperature of the developing-room. 
The plate may then be examined leisurely by placing it in 
front of, and at some distance from, a sheet of white paper. 
If it is not sufficiently black, add about 2 drops of silver 
nitrate solution to each drachm of developer, stir well with a 
glass rod, and continue the action until the requisite amount 
of intensity is obtained.” 


308 


THE CHEMISTRY OF PHOTOGRAPHY. 


Intensification in this way might really be styled re-develop- 
ment,” or “continued development.” It was effected ~before 
the plate was fixed. 

Intensification of Collodion Negatives after Fixing'. — In 
the second edition of Hardwicli (1855), he adds to the para- 
graph quoted above : “ When there is any disposition in the 
plate to fog towards the end of the development, it may some- 
times be obviated by fixing with cyanide of potassium as soon 
as the ‘ development proper ’ is complete, and then after a 
careful washing intensifying with pyrogallic acid and nitrate 
of silver in the usual way.” 

In the third edition (1856) of this classical book, the subject 
is put in a very masterly way : “ Mode of Increasing the Inten- 
sity of the Negative Image. — For the sake of clearness, we 
establish two stages in the development of a collodion nega- 
tive ; first, the development proper.^ or bringing out of an 
image distinct in all its details by transmitted light ; but pale 
and comparatively translucent ; second, the development hy 
precipitation^ as it has been termed, by which the image is 
rendered darker and more opaque. 

“ The strengthening of a feeble image is effected by pouring 
over the plate a mixture of pyrogallic acid and nitrate of 
silver. These two substances decompose each other even with- 
out the aid of light, and a deposit of silver is formed which 
settles down upon the image and adheres to it, * ^ * 

“The collodion image is sometimes spoken of as being 
within the substance of the transparent film. This, however, 
is incorrect ; it is really upon the surface of the film, and is 
formed by a superposition of metallic particles rather than by 
a penetration inwards. The mere act of varnishing the plate 
will often prove this to be the case ; the elevated lines of the 
impression being seen to form an obstruction to the flow of 
the spirit, and so to produce a series of permanent ridges at 
various parts of the plate.” 

This is one of the main points of difference between the 
gelatine and the collodion processes. In the gelatine drj"- 
plate, the silver image is within the film, and forms from the 
surface downwards. But silver is added to the collodion film 


INTENSIFYING PROCESSES AND THEIR CHEMISTRY. 


309 


during or after development to strengthen tlie image (which 
is upon the surface only), and this added silver intensifies or 
builds up upon the delicate surface-image which has been 
produced by light. 

In the last (ninth) edition of Hardwich, which was published 
in 1883, we get a masterly summiiig.up of the whole matter 
as follows (page 128) : “ The Second Stage of the Develojpment. 
This consists in strengthening the image first formed, by an 
additional deposit of silver. Take a sensitive collodion plate, 
and having impressed an invisible image upon it by a proper 
exposure in the camera, remove it to the dark-room, and pour 
over it a solution of pyrogallic acid. When the picture has 
fully appeared, stop the action by washing the plate with 
water. An examination of the image at this stage will show 
that it is perfect in the details, but pale and translucent. 

“ iN’ow take the plate and treat it with pyrogallic acid to which 
fresh nitrate of silver has been added ; immediately the picture 
will become much blacker, and will continue to darken even 
to complete opacity, if the supply of nitrate be kept up. The 
same result may be obtained after the iodide of silver has been 
removed from the plate by hyposulphite of soda or cyanide 
of potassium ; and in such a case it is evident that the addi- 
tional deposit upon the image must be produced from the 
nitrate of silver, and not from the iodide of silver. Observe 
also, that this additional deposit only ujpon the image^ 
exhibiting no affinity for the unaltered iodide upon the part of 
the plate corresponding to the shadows of the picture, but 
attaching itself in preference to those parts already blackened 
by the developer. 

“ The second stage of the development, in which a feeble 
image is strengthened and rendered more opaque, is a process 
bearing a close resemblance to the growth of a crystal in a 
saturated liquid by aggregation of fresh particles ; and after 
the picture has reached its full density, a series of elevations 
may often be seen upon the plate, corresponding to the lines 
of the image.'’ 

The chemistry of intensification with silver is identical with 
that of development. The silver is reduced to the metallic 


310 


THE CHEMISTRY OF PHOTOGRAPHY. 


state, and is attracted by and deposited upon the already 
existing image. The method is — as we have already pointed 
out — only a continuation of development. 

Intensification of Collodion Plates with Mercury, — The 
curious effect of tlie compound of mercury and chlorine known 
as mercuric chloride, or bichloride of mercury (Hg Cl 3 ) was 
noticed very early in the history of jihotography. 

In an important paper “ On the Chemical Action of the 
Rays of the Sun,” etc., communicated by Sir John Ilerschel 
to the Royal Society^* in 1840, he writes : “ By far the most 

remarkable fixing process with which I am acquainted, how- 
ever, consists in washing over the picture with a weak solution 
of corrosive sublimate,f and then laying it for a few moments 
in water. This at once and completely obliterates the picture, 
reducing it to the state of perfectly white paper, on which the 
nicest examination (if the process be perfectly executed) can 
detect no trace, and in which it can be used for any other pur- 
pose, as drawing, writing, etc., being completely insensible to 
light. Nevertheless, the picture, though invisible, is only dor- 
mant, and may be instantly revived in all its force by merely 
brushing it over with a solution of a neutral hyposulphite, af- 
ter' which it remains as insensible as before to the action of 
light. And thus it may be successively obliterated and revived 
as often as we please. It hardly requires mention that the 
property in question furnishes a means of painting in mezzo- 
tinto {i.e. of commencing on black paper and working in the 
lights), as also a mode of secret writing, and a variety of simi- 
lar applications.” 

This discovery by Ilerschel contains the foundation of all 
that was done afterward with mercury bichloride as an intensi- 
fying agent. It is true that Ilerschel hardly recognizes that 
an increase in the intensity of the picture is produced by his 
method, although the vvmrds, “revived in all its force f show 
that he was impressed by the vigor of the results. 

It may be thought that this important paper by Ilerschel 
was “ buried,” and inaccessible to most photographers in the 


“ Philosophical Transactions,” Part I. for 1840 ; p. 1. 
t This is the common or trivial name for mercuric chloride. 


INTENSIFYING PROCESSES AND THEIR CHEMISTRY. 311 

medium (Phil. Trans.) in which it was published ; but most 
of its facts were utilized (with due acknowledgment) in Hunt’s 
well-known text-book, Kesearches on Light,” the first edition 
of which appeared in 1844. Hunt repeats and extends Her- 
schel’s experiments with mercury bichloride. 

In the year 1840 Eobert Hunt contributed a paper to the 
Philosophical Transactions of the Eoyal Society,”* in which 
he describes sundry attempts that he had made to obtain 
daguerreotypes upon He says : “If one of the above 

papers, when removed from the mercurial vapor, be dipped 
into solution of mercury bichloride, the drawing disappears ; 
but after a few minutes it is seen, as if by magic, unfolding 
itself, and gradually becoming far more beautiful and whiter 
than before ; delicate lines, before invisible or barely seen, are 
now distinctly marked, and a rare and singular perfection of 
detail given to the drawing.” Herschel’s pictures became, and 
rew^ained invisible after treatment with mercury bichloride 
because a white image was produced upon a white surface ; but 
Hunt used a black-surfaced paper, so that the white image was 
distinctly visible upon the black background. 

In the first edition of “ Hunt’s Text Book of Photography,” 
the preface of which is dated July, 1851 (and which must 
therefore have been written to Archer’s communication 

to the Athenmim, which we notice further on), he carries the 
idea of the use of bichloride of mercury as an intensiher a 
little further. Hot that he actually describes its use for this 
pur]30se ; but he gives notes from which other workers could 
doubtless get the idea of using the mercury salt for the pur- 
pose of intensification. He writes (p. 190) : “ Hip one of 

the daguerreotype pictures, formed on the sulphuretted paperf 
into a solution of corrosive sublimate ; the drawing instantly 
disappears, but, after a few minutes it is seen unfolding itself, 
and gradually becoming more distinct than it was before, 
delicate lines, before invisible, or barely seen, are now distinctly 
marked, and a rare and singular perfection of detail given to 
the drawing.” 


* Vol. for 1840 ; p. 325. 

+ The “sulphuretted paper” was paper blackened with sulphide of silver. — 'W. J. H. 


312 


THE CHEMISTRY OF PHOTOGRAPHY. 


Frederich Scott Archer Publishes a Mercury- Hypo Process 
of Intensification in 1851. — Arclier did three notable pieces of 
work in photography in the year 1851. First, of conrse, 
comes the collodion process itself, which he announced in The 
Chemist^ in March, 1851. Next, he showed that pyrogallic 
acid was superior to gallic acid as a dcYeloping agent ; and in 
the lasfc month of the year he described a process of intensifi- 
cation which (slightly altered by the substitution of ammonia 
for hypo) is the most frequently employed of any at the pres- 
ent day. 

In the Athenmim for December 20, 1851, p. 1350, Archer 
writes : I wish to communicate a peculiar process of whiten- 

ing and blackening the collodion pictures, which may possibly 
prove interesting. 

The picture being thoroughly washed in plenty of water, 
after fixing with hyposulphite of soda, is . treated in the follow- 
ing manner: Prepare a saturated solution of bichloride of 
mercury in muriatic^' acid. Add one part of this solution to 
six of water ; pour a small quantity of it over the picture at 
one corner, and allow it to run evenly over the glass. It will 
be found immediately to deepen the tones of the picture con- 
siderably, and the positive image will almost entirely dis- 
appear ; but presently a peculiar whitening will come on, and 
in a short time a beautifully delicate white picture will be 
brought out.f The negative character of the drawing will be 
almost entirely destroyed, the white positive image alone 
remaining. This picture, after being well washed and dried, 
can be varnished and preserved as a positive ; but nevertheless, 
e\ en after this bleaching, it can be changed into a deep-toned 
negative, many shades darker than it was originally, by im- 
mersing it, after a thorough washing, in a weak solution of 
liyposulphite of soda. In a short time the white picture will 
entirely disappear, and a black negative image will be the 
result. It is very singular that the jficture can be alternately 
changed from white positive to black negative many times in 

This was the name by which hydrochloric acid was formerly known.— W. J. H. 
t Under the name of the “ alabastrine process ” such pictures became quite the vogue a 
few years later. — W. J. H. 


INTENSIFYING PROCESSES AND THEIR CHEMISTRY. 313 


succession, and very often with improvement to the picture. 
By the above process a most perfect white or a deep black 
negative picture can he obtained, quite distinct from each 
other.” 

In the French photographic journal. La Lumiere^ for 24th 
September, 1853, M. Disderi describes the application of the 
above process to the intensification of negatives on pa2)er, 
which was accomplished, he says, with complete success. 

Hunt Avrote to the Atlienceum (p. 23, for Jan., 1853), claim- 
ing that both Herschel and himself had already published a 
similar process to that described by Archer. The latter, in 
reply (p. 87), states that he was unaAvare of Hunt’s paper in 
the Phil. Trans. ; and that his process, being upon collodion, 
was difierent to Hunt’s, which Avas upon paper. 

j\Iaconochie Deejyens^"^ Negatives with Gold. — In the 
Photograjphic Journal for August, 1853, the following pro- 
cess, devised by Professor Maconochie, of GlasgoAv, is headed 

Method of Deepening hlegatives.” To 1 ounce of distilled 
water are added 3 grains each of ammonium chloride and gold 
chloride. The developed and washed (Avet collodion) plate 
darkens rapidly when this solution is poured over it. In the 
same periodical for February, 1856, Mr. Titterton recommends 
the gold solution to be applied to a plate which has already 
been intensified Avith mercury, if a considerable increase of 
density be desired. 

The chemical change which takes place is, of course, the 
substitution of gold for silver. 

3Ag + AuClg — Au + 3AgCl 

Silver and Gold Chloride produce Gold a7id Silver Chloride. 

Halleur Combines Intensification with Fixing in 1853. — 
In the German text-book of photography Avritten by Dr. Hal- 
leur in 1853, and of which a translation was published in Eng- 
land in 1851, we are fold (p. 44) that in the calotype process 
“the picture may be fixed also by washing it with a solution 
of chloride of mercury (corrosive sublimate), rinsing it subse- 
quently in water and letting it dry. This operation renders 
the picture perfectly invisible, and leaves, in the case of silver 


314 


THE CHEMISTRY OF PHOTOGRAPHY. 


cliloride paper, a white, in that of iodide paper a yellowish 
surface. But the invisible picture may be brought to light at 
any time by washing with a solution of hyposulphite of soda, 
rinsing in water, and drying.” 

The collodion process is not mentioned in this work ; it had 
probably hardly reached Grermany at the time the book was 
written. 

Maxwell-Lyte Intensifies Collodion Negatives with Mercury 
and Potassium Iodide. — In the first volume of the Journal 
of the (London) Photographic Society.^ published in 1853, one 
of the leading amateurs of the day, Mr. F. Maxwell-Lyte, 
writes (p. 128): 1 first of all whiten the picture by means of 

the solution of bichloride of mercury in hydrochloric acid, of 
which, according to Archer’s method, I take one part to six of 
water; and then, after well washing the plate, I pour on a 
weak solution containing about 2 grains to the ounce of iodide 
of potassium ; by this means a fine yellow picture is produced 
quite impervious to actinic rays.” 

The change from white to yellow would, in this case, be 
produced by the conversion of the white salt of mercury (the 
bichloride) into a yellow salt according to the following 
equation : 

HgCl, + 2KI = Hglg • + 

Mercuric Chloride and Potassium Iodide prodtice Mercuric Iodide and 

2KC1 

• Potassium Chloride. 

The yellow compound obstructing especially the rays (blue 
and violet) which are most effective, the negative is intensified 
correspondingly. 

Donny uses Mercury followed hy Sulphuretted Hydrogen^ 
in 1853. — After the appearance in the Photographic Journal 
for 1853 of Maxwell-Lyte’s iodide process of intensification 
(which we have just described), a corres]3ondent wrote (F. 
Hudson, p. 164), complaining of difficulties caused by the 
iodide of mercury being soluble in the other solutions employed. 
This called forth a letter (p. 186) from Professor F. Donny to 
the following effect: '‘ During last summer I converted into 
very dark black negatives a good many instantaneous collodion 


INTENSIFYING PROCESSES AND THEIR CHEMISTRY^ 


315 


positives, by means of the following process : After develop- 
ment the picture is washed, drained, and immediately whitened 
according to Archer’s metliod ; being carefully washed again 
with ] ain-water and drained, but not dried, I cover it with a 


solution of ^ 

Gum arabic— by weight 1 part 

Water — by weight 10 parts 


“ Whilst this gummy covering is still moist, the picture is 
exposed, in a vertical position, to a strong current of sulphur- 
etted hydrogen, which converts it rapidly into a black nega- 
tive. The operation is then at an end, and the picture is set 
up to dry ; nothing of the former positive appearance is to be 
seen on it, even when the glass side is turned towards the eye. 

In this way black negatives of the utmost darkness are 
obtained, and will prove much more satisfactory than the 
yellow ones procured by the iodide process.” 

AVhat is the chemical change which takes place by this 
method ? 

HgClg + SHg = FtgS 

Mercuric Chloride and Sulphuretted Hydrogen produce Mercuric Sulphide 

+ 2HC1 

a)td Hydrochloric Acid. 

It is hardly necessary to remark that the offensive smell of 
sulphuretted hydrogen (“rotten-egg gas”) would deter most 
photographers from even testing this method. 

Mercury followed hy Ammonia as an Intensifier^ used hy 
Hurd for Collodion Negatives in 1853. — The first notice 
which we have been able to find of the most commonly 
adopted process of intensification of the present day — mercury, 
followed by ammonia — is contained in the third edition of 
“Hunt’s Manual of Photography.” This edition bears the 
date 1853 on its title-page ; but as the preface is dated Decem- 
ber, 1852, the discovery must have been made during the 
latter year. 

Writing of collodion negatives, Hunt says (p. 268): “A 
peculiar whitening process was introduced by Mr. Archer, 
which is as follows : The picture being thoroughly w^ashed in 
plenty of water, after fixing with hyposulphite of soda, is 


316 


THE CHEMISTRY OF PHOTOGRAPHY. 


treated in the following manner : Prepare a saturated solution 
of bichloride of mercury in muriatic acid. Add. one part of 
this solution to six of water. Pour a small quantity of it over 
the picture at one corner, and allow it to run evenly over the 
glass. It will be found immediately to deepen the tones of 
the picture considerably, and the positive image will almost 
disappear ; presently, a peculiar whitening will come over it, 
and in a short time a beautifully delicate white picture will 
be brought out. 

“ The negative character of the drawing will be entirely 
destroyed, the white positive alone remaining. This picture, 
after being well washed and dried, can be varnished and pre- 
served as a positive ; but, nevertheless, even after this bleach- 
ing, it can be changed into a deep-toned negative, many shades 
darker than it was originally, by immersing it, after a thor- 
ough washing, in a weak solution of hyposulphite of soda, or 
a weak solution of ammonia. The white picture will vanish, 
and a black negative will be the result. 

‘‘It is very singular that the picture can be alternately 
changed from a white positive to a black negative many times 
in succession, and very often with improvement. 

“ Thus, by the above process, a most ]3erfect white positive, 
or a deep black negative is produced, quite distinct from each 
other. 

‘‘ In the first part of this after-process it will be observed that 
the effect of this bichloride of mercury solution is to deepen 
the shades of the picture, and this peculiarity can be made 
available to strengthen a faint image, by taking the precaution 
of using the solution weaker, in order that the first change 
may be completed before the whitening effect comes on. The 
progress of the change can be stopjDed at this point by the 
simple application of water.’' 

The chemical changes produced during this method of 
intensification are explained further on, when treating of the 
process as applied to gelatine negatives. 

Intensification according to Ilardwich . — The nine editions 
of llardwich’s “ rhotogra[ hie Chemistry” (1855-83), forma 
sort of guide to photographic progress. 


INTENSIFYING PKOCESSES AND THEIR CHEMISTRY. 317 

In the first edition (1855), we have two pages on “ The 
Means Employed to Strengthen a Finished Impression which 
is too feeble to be used as a iSiegative.” It is pointed out 
that the plan of pushing,” or re-developing, which we 
have already described as performed with pyro and silver, 
‘‘cannot be ajDplied with advantage after the picture has been 
washed and dried.” Three plans of intensification are then 
given : 

(1) Bonny’s Method^ with mercury bichloride followed by 
sulphuretted hydrogen or hydrosulphate of ammonia. 

(2) Barreswil and Bavannds Process by which the 
image is converted into iodide of silver, by treatment with 
iodine, exposed to light, and then re-developed. 

(3) Hunds Method^ with mercury chloride followed by 
ammonia. 

The second (1855) and third (1856) editions of Hardwich 
show no change ; but the fourth (1857) adds cyanide of 
potassium as a substance which may be used to blacken the 
image after the application of mercuric chloride. 

The sixth edition (1861) states (p. 170) that “the writer 
dispenses entirely with the employment of the bichloride of 
mercury, and acts on the image with a solution of iodine in 
iodide of potassium until it is converted into iodide of silver, 
after which the hydrosulphate of ammonia is applied in the 
usual way.” 

The ninth and last (1883) edition gives sulphide of ammo- 
nium, and cyanide of silver dissolved in cyanide of potassium, 
as other substances which may be used to blacken the white 
image produced by the application of mercury bichloride. 

Intensifying with Platinum. — Immerse the negative in a 
solution of platinum tetra-chloride, of the strength of about 
twenty grains to the ounce. The following change then takes 
place : 

PtCl4 + 2Aga =: Pt + 4AgCl 

Piatinum and Silver produce Platinum and Silver 

Chloride Chloride. 

* This appeared in the Chijnie Photographique^ and was translated in the Photographic 
Journal for August, 1854. The method is strongly recommended by R. J. Fowler in the 
'same Journal for May, 1857. 


318 THE CHEMISTRY OF PHOTOORAPHY. 

The silver of the original image thus changes place with 
the platinum. 

The Platinotjpe Company sell a one-solution intensifier 
which Captain Abney states ‘‘ is composed of mercuric chlo- 
ride and a salt of platinum.” It acts by changing the image 
to an orange-brown color. 

Schlijppe’s Salt as an Intensifier (Carey Lea). — In the year 
1865, Mr. Carey Lea, the famous American photo-chemist, 
announced* a method of intensification by the use of Schlippe’s 
salt (sodium sulphantimoniate), which has since proved of 
service, especially where a considerable increase in the opacity 
of the negative is desired. As recommended by Lea for col- 
lodion negatives, the method consisted in converting the silver 
of the image into silver iodide, which was then reddened by 
the Schlippe’s salt. For gelatine negatives it is better to con- 
vert the silver into silver chloride by soaking the negative in 
a bath of ferric chloride (say twenty grains to the ounce) : 

FegCle + Aga 2AgCl + 2FeClg 

Ferric and Silver produce Silver and Ferrous 

Chloride Chloride Chloride. 

Now wash the negative thoroughly, and immerse it in a 
bath of Schlippe’s salt (strength, say a saturated solution, 
diluted with an equal volume of water) when the image will be 
converted into silver sulphantimoniate, which is of a scarlet 
hue. 

3AgCl H- NaaSbS 4 = AggSbS^ + 3NaCl 

Silver and Schlippe’s produce Silver Sul- and Sodium 

Chloride Salt phantimoniate Chloride. 

The scarlet substance is very opaque, and the intensification 
is correspondingly considerable. The process is, in fact, better 
suited for reproduction of line engravings, etc., than for land- 
scape negatives. 

The negative must finally be washed and dried. 

Intensifying Collodion Negatives with Quinol. — In 1888 
Captain Hubl recommended f the use of quinol (hydro- 
quinone) for intensifying collodion negatives, as follows : 


'^'British Journal of Photography^ Vol. XII., pp. 55, 288. 
t See paper by Dr. Eder in Photographic Neivs for January 3, 1890. 


INTENSIFYING PROCESSES AND THEIR CHEMISTRY. 


319 


Solution A. 


Hydroquinone 10 parts 

Citric acid 6 parts 

Water 1000 parts 

Solution B. 

Nitrate of silver 1 part 

Water 30 parts 


For use, mix 3 ounces of a A with 1 of B. 

Intensification of Gelatine Negatives : I. — Intensifiers 

Containing Mercury. 

The collodion process was displaced — for general work — by 
gelatine in the years 1879 to 1881. 

Gelatine dry-plates consist of an emulsion of silver brcanide 
in gelatine, spread upon glass or celluloid. By exposure to 
light, the silver bromide suffers a chemical change, though the 
precise nature of that change has not yet been ascertained with 
certainty. The only point in dispute, however, is as to whether 
the non-metallic element bromine is separated altogether, or 
only in part (and if in part, how much ?) from the metallic 
silver. For simplicity let us here supjiose that the effect of 
light is to form a picture in metallic silver upon the surface of 
the gelatine emulsion. This picture is so weak that it is in- 
visible ; but when the developer (usually an alkaline solution 
of pyrogallic acid) is poured over the plate, it enables the re- 
duced silver to attack and decompose the silver bromide lying 
beneath it ; and thus the image grows downwards^ and becomes 
visible at last when the back of the plate is examined. This 
is just the opposite of the action of development on a collodion 
plate. On the latter the image is built up from without, and 
upwards ; on the gelatine plate from within, and the image 
grows downwards. 

Negatives on Gelatine Plates Frequently Peguire Intensi- 
fying. — After a gelatine dry-plate has been exposed in the 
camera, and then developed, fixed, and washed to the best of 
the manipulator’s ability, it is frequently found to give a very 
unsatisfactory print. In this case intensification may effect an 
improvement. It is a good plan always to take a print from 


320 


THE CHEMISTRY OF PHOTOGRAPHY. 


any negative before intensifying it. Many negatives print 
better than they look ; and in any case the print affords a means 
of subsequently estimating what improvement — if any — has 
been effected. 

There are three principal causes for which intensification is 
supposed to offer a remedy. 

1. Under-development. 

2. Under-exposure. 

3. Over-exposure. 

We believe, however, that it is only in the last case, viz : 
thinness from over-exposure — that the intensifying process 
offers any real advantage. In any case the best remedy is — 
to take another negative. But where, from moderate over- 
exposure, a negative shows a delicate, thin image, full of detail, 
it may, by intensification, be made to yield a passable print. 
This is one reason why all the text-books agree in recommend- 
ing workers generally to err — if in doubt — on the side of over- 
exposure. 

Intensification of Gelatine Negatives with Mercury 
Bichloride; followed by Ammonia or some other 
Darkening Agent. 

1. Intensification by Mercury Bichloride Alone. — Make’ up 


the following solution : 

Mercury bichloride (corrosive sublimate) ^ ounce 

Ammonium chloride (sal ammoniac) ^ ounce 

Hydrochloric acid 10 minims 

Distilled water 10 ounces 


Dissolve the sal ammoniac in the acid water ; then powder 
the corrosive sublimate in a mortar, and add it to the solution. 
Shake well at intervals, and allow to stand for a few hours ; 
then filter. The addition of the sal ammoniac enables the 
water to more readily dissolve the mercurial salt. The bottle 
should be labelled 

The negative to be intensified should be thoroughly washed 
after being developed ; it should then be soaked for half an 
hour in an alum bath ; and then washed again in running 
water for one hour. It must then be allowed to dry. 


INTENSIFYING PROCESSES AND THEIK CHEMISTRY. 321 


It is a good plan to keep one dish — a glass one is to be pre- 
ferred — specially for the work of intensification, as the solu- 
tion employed will injuriously atfect both negatives and prints 
which do not require its aid. 

Soak the dried negative in water for ten minutes, and then 
place it in the mercury solution. Eock the dish gently. The 
image steadily whitens, until at last it becomes clearly visible 
as a beautiful positive. The chemical change is expressed by 
the following equation : 

Agg + 2HgClg — 2AgCl + 

Silver and Mercuric Chloride produce Silver Chloride and 

HggClg 

Mercurous Chloride. 

Thus the white substance of which the image is now com- 
posed is a mixture of silver chloride and mercurous chloride 
(commonly called calomel). 

The image, after this process, is slightly stronger and denser 
than before. But, being composed of white and somewhat 
translucent matter, the print which it now yields is only a 
slight advance in the above respects over that given by the 
original unintensified plate. 

By acting upon the whitened image with one or other of 
several re-agents, it is possible to change its color to one which 
shall better obstruct the rays of light. The intensification will 
then be much more marked. 

Blackening ivith Ammonia . — The plate which has been 
whitened by the mercury bichloride must receive a very 
thorough washing if the next process which it has to undergo 
is to be productive of permanent results. Binse it thoroughly 
in two or three changes of water, and then wash it in running 
w^ater for at least half an hour. The mercurial salt is much 
more soluble in water to which a little ammonium chloride 
has been added, than in water alone. Therefore soak the 
negative for ten minutes in water, 5 ounces, ammonium chlo- 
ride, J ounce. 

While this is being done, prepare the following solution : 


Ammonia (strong) 2 drachms 

Water 10 ounces 


322 


*TUE CHEMISTRY OF PHOTOGRAPHY. 


Place this alkaline solution in a clean dish, and immerse the 
washed and whitened negative therein. Its color quickly 
changes — first to brown and then to black. ‘What is the cause 
of this ? 

HggClg + SNHg = 2NH3HgCl 

Calomel and Ammonia produce Mercurous-ammonium 

Chloride. 


The silver chloride is dissolved by the ammonia, and is 
washed away. 

The image is now weaker in point of quantity of material, 
than it was before the application of the ammonia ; for the 
silver chloride has been removed. But the change of color 
has made it more opaque. If the negative be now washed for 
live minutes, and then dried, it will (supposing it to have 
been thin and over-exposed to begin with) probably yield a 
much better print than before this process of intensification 
was carried out. 

If twice the quantity of ammonia named above be used (4 
drachms instead of 2), a somewhat blacker negative will be 
obtained. 

If the application of ammonia produces, or is followed by 
spots and stains, it is a sign that the negative has not been 
thoroughly freed from hypo by washing. 

Blackening hy Sodium Sulphite . — Instead of using ammo- 
nia, the whitened negative may be changed in color by 
immersion in a saturated solution of sulphite of soda, to 
which half its bulk of water, and two grains per ounce of 
citric acid, have been added. 

Crush two ounces of clear crystals of sodium sulphite iii a 
mortar and add eight ounces of water. This ought to just 
dissolve the solid sulphite. Then add four ounces more of 
water, and shake well. This solution should be used soon after 
it has been prepared. The chemical action is now as follows : 


HgaCl^ + NagSOg + HgO = 2Hg + 

Calomel and Sodium and Water produce Mercury and 

Sulphite 

Na^SO^ -f 2HC1 

Sodium and Hydrochloric 

Sulphate Acid. 


INTENSIFYING PROCESSES AND THEIR CHEMISTRY. 323 

So tliat an image in black reduced mercury is obtained. 
Our own experience with this intensifier is that it imparts 
less density than ammonia. It is therefore a good intensiher 
for negatives that only require a sliglit strengthening. 

Blackening hy Ammonium Sulphide. — When a very con- 
siderable increase of density is required, the whitened nega- 
tive may be soaked in the following solution : 


Ammonium sulphide 1 part 

Water 20 parts 


The ammonium sulphide is a yellow liquid possessing a 
very disagreeable smell. Its effect is to convert hoth the chlo- 
rides of which the whitened image is composed into their cor- 
responding sulphides ; and these are very black and opaque. 
Take first the action upon the mercurous chloride : 

Hg^Cl, + {NU,),S = Hg,S + 2NH4CI 

Calomel and Ammonium produce Mercurous and Ammonium 

Sulphide Sulphide Chloride. 

A similar change is produced with the silver chloride : 

2 AgCl + (NH4)2S = AggS + 2NH4CI 

Silver and Ammonium produce Silver and Ammonium 

Chloride Sulphide Sulphide Chloride. 

This powerful intensifier is apt, however, to block up and 
destroy the half-tones. It is well suited, however, for copies 
of printed matter, etc., in - which a dense black-and-white 
negative is desired. 

Blackening hy P otassio- Ferrous Oxalate. — To eight ounces 
of a cold saturated solution of potash oxalate, add two ounces 
of a cold saturated solution of ferrous sulphate. This is the 
ordinary “ ferrous oxalate ” developer. Its use in intensifica- 
tion was first proposed by Messrs. C. I. Burton and A. B. 
Laurie. (See British Journal of Photography for 1881, pp. 
287, 294.) 

When the whitened negative is soaked in the above mix- 
ture, the haloid salts are quickly reduced to the metallic form, 
and we get the original silver image back again, plus an image 
in mercury. 


324 


THE CHEMISTRY OF PHOTOGRAPHY. 


The effect upon the silver haloid is as follows: 

2AgCl + 2FeCgO, + = Ag^ + 

Silver and Ferrous and Potash produce Silver and 

Chloride Oxalate Oxalate 

Fe2(C,04)3 + 2KC1 

Ferric and Potassium 

Oxalate Chloride. 

Upon the mercurous chloride (calomel) a similar effect is 
produced : 

HggClg + 2 FeCa 04 + KgC^O^ = 

Calomel and Ferrous Oxalate and Potash Oxalate produce 
2Hg + + 2KC1 

Mercury and Ferric Oxalate and Potassium Chloride. 

The mixture of the two metals (silver and mercury) in a 
finely-divided state, gives a dark and opaque image. Additional 
intensity can be imparted by whitening the negative a second 
time with mercury bichloride, and then repeating the opera- 
tion with ferrous oxalate as before. 

Blackening with Potassio-Silve?^ Cyanide , — Dissolve 120 
grains of silver nitrate in 10 ounces of distilled water ; and add 
to it, drop by drop, a strong solution of potassium cyanide, 
until the white precipitate at first formed is just dissolved 
(stir with a glass rod). The solution ought then to look 
opalescent, or as if a drop of milk had been added to the 
water. Soak in it the negative which has been bleached with 
mercury bichloride (as already described) until it is quite 
black, as seen from the back. Then wash for an hour, and 
dry. This method gives a considerable intensification, with 
very little blocking-np of detail. It is one which we have 
practised with much success. 

Professor Meldola believes that the blackening is due to the 
following chemical reaction : 

Hg^Cl^ + 2AgK(CN)2 = Ags + 

Calomel attd Potassio Silver Cyanide produce Silver and 

2IIg(CN)2 + 2KC1 

Mercuric Cyanide and Potassium Chloride. 

The blackened image is therefore composed of metallic silver 
and mercuric cyanide. 


INTENSIFYING PROCESSES AND THEIR CHEMISTRY. 325 

This method is due to the late Dr. Yan Monckhoven. 

Blackening with Ilydroquinone. — In 1889 Dr. Mailman, in 
Germany, and A. D. Dresser, in England, recommended the 
ordinary developing solution of hydroquinone, as made up v^ith 
sulphite of soda, to blacken and intensify negatives which had 
been treated with mercury bichloride. The following solution 
answers well : 

Hydroquinone. 20 grains 

Sulphite of soda 80 grains 

Distilled water 5 ounces 

This gives a fine bluish-black color to the negatives. 

Ik tensifying Gelatine Negatives with Mercuric Iodide jplus 
Hypo {EdwardI Intensifier). — In the British Journal 
Almanac for 1880 (p. 56), the well-known plate-maker, Mr. 
B. J. Edwards, published a method of intensification which 
has been much used since. 

Its latest form is as follows : 

Dissolve 60 grains of mercury bichloride in 8 ounces of 
water. Add to this enough potassium iodide to neaidy re- 
dissolve the red precipitate which is at first formed (about 150 
grains will be required). Lastly, add 120 grains of hypo- 
sulphite of soda in crystals, and shake well. This should give 
a clear solution : 

HgCU + 2KI = Hglg 

Mercury Bichloride and Potassium Iodide produce Mercuric Iodide 

-F 2KCI 

and Potassium Chloride. 

The negative to be intensified need be only slightly rinsed 
or washed before transfer to the above solution. It will there 
quickly gain in printing density. 

Lastly, place the intensified negative in a weak fixing-bath 
(2 ounces hypo to 20 of water), for a quarter of a minute (not 
more), and then wash well, and dry. 

Intensification with Mercuric Bromide followed hy Ferrous 
Oxalate. — Messrs. C. I. Burton and A. P. Laurie described in 
1881''" how to intensify gelatine negatives as follows : 


* British Journal of Photography^ pp. 287, 294. 


326 


THE CHEMISTRY OF PHOTOGRAPHY. 


Solution A. 


Mercuric bromide 1 part 

Water 250 parts 


This is a saturated solution. 

Solution B. 


Ferrous sulphate (sat. sol.) 1 part 

Potash oxalate “ 2 parts 


Pour the sulphate into the oxalate, and not vice versa. The 
mixture is the ordinary ferrous oxalate developer. 

Bleach the negative in A ; then wash it, and expose it to 
sunlight for two or three minutes. 

Then apply the developer (B), just as if developing a nega- 
tive. The image blackens, and gains considerably in density. 
If still greater density be required, the whole process may be 
repeated. The plate must be left ten or twenty minutes in 
the developer in order to thoroughly blacken it. It must then 
be washed and dried. 

The chemistry of this method will be similar to that where 
mercuric chloride is used, followed by ferrous oxalate. 

Gelatine Negatives Intensified hy Mercuric Iodide^ followed 
hy BcJdijyjyd s Salt. — The following method was given in an 
editorial article in the PJiotograjghic News for 15th July, 
188J: 

Make a solution of mercuric iodide by adding a strong (one 
in five) solution of potassium iodide to a saturated solution of 
mercury bichloride. About ounces of the former to 10 
ounces of the latter will be right. The red precipitate which 
forms should just re-dissolve. For use, add 3 ounces of water 
to each ounce of the above mixture : 

HgClg + 2KI = Hgig 

Mercury Bichloride and Potassium Iodide produce Mercuric Iodide 

+ 2KC1 

and Potassium Cliloride. 

Soak the plate to be intensified in this solution until it is 
nearly (but not quite) dense enougli. Then wash well for one 
hour. 

Make a solution of Sclilippe’s Salt (properly called sulph- 


INTENSIFYING PEOCESSES AND THEIE CHEMISTEY. 


327 


aiitimoniate of soda) in water of the strength of five grains 
per ounce. Soak the washed negative in this until the desired 
density has been obtained ; then wash well and dry. 

Intensification of Gelatine Negatives. II. — Intensifying 

WITHOUT IVIeECIJEY. 

Wellington^ 8 Silver Intensijier for Gelatine Negatives . — In 
the Photo. Almanac” for 1889 (p. 575), Mr. J. B. B. Wel- 
lington writes : Silver intensification as used for wet (collo- 

dion) plates, namely, with nitrate of silver and pyro, is out of 
the question for the ordinary work of the photographer of the 
present day, as the hypo has to be thoroughly eliminated from 
the gelatine film by long-continued washing, and even after 
this has been done the nitrate of silver has often a persistent 
habit of staining the film red, and which occurs even in collo- 
dion plates. 

I can now carry on intensification without the silver being 
thrown out of solution, producing a negative of any intensity 
from the merest ghost of an image, and resembling in charac- 
ter any ordinarily developed negative : 

Silver nitrate 100 grains 

Distilled water 2 ounces 

‘^Add to this 240 grains of sulphocyanide of ammonium ; a 
precipitate is formed which is again dissolved. On diluting 
this to ten ounces wnth water another precipitate is thrown out. 
Now dissolve this precipitate by adding hyposulphite of soda 
to the solution. This constitutes the stock solution. 

To intensify take — 


Stock solution 1 ounce 

‘^And add — 

Pyro 3 grains 

Sulphite of soda 12 grains 

Ammonia 6 minims 

Ammonium bromide 2 grains 


From five to ten minutes will produce a dense negative 
from a very thin one witliout staining in the slightest degree. 


328 


THE CHEMISTRY OF PHOTOGRAPHY. 


More ammonia may be added from time to time if not suffi- 
ciently energetic. For wet-plates, collodio-bromide, and gela- 
tine, it cannot be surpassed at present.” 

Intensifying with Uranium {Selle). — In the Bulletin Beige 
de la Photographies for 1865, M. Hermann Selle showed 
how to intensify collodion negatives with a mixture of sul- 
phate of uranium and cyanide of potassium and iron ; and 
in 1866 Duncan substituted the nitrate of uranium for the 
sulphate. Lastly, in his classical book on Modern Dry- 
Plates” (1881), Eder showed how useful the method was for 
gelatine negatives. 

Make up the following solution : 


Uranium nitrate 20 grains 

Potassium ferridcyanide 25 grains 

Distilled water 7 ounces 


, This solution should be perfectly clear. The negative must 
be soaked in it until it is of a brownish-red color ; and then 
we'^l washed. 

The uranium and potassium salts combine to form uranium 
ferricyanide, and this last named substance combines with the 
silver (of the image) to produce ferrocyanides of silver and 
uranium. The uranium ferrocyanide being naturally of a 
dark-brown color, it is not necessary to use any blackening 
agent such as is needed in intensifying with lead. Otherwise, 
the chemical changes which take place are similar, and may 
be expressed by similar equations. 

Intensifying with Lead {Eder and Toth). — In the year 
1S76, two Austrian investigators — Professor J. M. Eder and 
Captain Y. Toth — pubhshedf a method of intensifying collo- 
dion negatives with lead; the method is also applicable to 
gelatine negatives. 

In the first place it is necessary to prepare ferricyanide of 
lead, by mixing together the following substances : 


Nitrate of lead 4 parts 

Red prussiate of potash 6 parts 

Distilled water 100 parts 


* Translated in Photographic IVews, 1865, pp. 366, 498 ; and 1866, pp. 169. 202. 
t Photographic News, pp. 100, 573, 5T9, 593, 608. 


INTENSIFYING PROCESSES AND THEIR CHEMISTRY. 


329 


A cliemical change takes place, resulting in the formation 
of ferricyanide of lead : 

3Pb(N03)3 + K6Fe2(CN)i2 = PbsFeglCN)!^ + 

Lead Nitrate and Red Prussiate of Potash produce Lead Ferricyanide and 

Potassium Nitrate. 

Filter the mixture and immerse in it the negative to be 
intensified. The time required to produce the necessary den- 
sity is much longer if the negative has been previously dried ; 
it may then take hours. In this case the silver (of the image) 
combines with the lead ferricyanide to form the ferrocyanides 
of lead and of silver. 

2Aga + 2Pb3Fe3(CN)i3 = Ag4Fe(CN)c + 

Silver and Lead Ferricyanide prodtice Silver Ferrocyaaide a7id 

3Pb3Fe(CN)6 
Lead Ferrocyanide. 

These ferrocyanides are white. To blacken them (thereby 
increasing their opacity) the plate must be well washed^ and 
then immersed in ammonium sulphide diluted with five times 
its volume of water. The white ferrocyanides are thereby 
converted into the dark sul2)hides of lead and of silver. 

ITiis method gives three times greater density than that 
obtained by the use of mercury and ammonia. It is seldom 
used at the present day except for line ” work. 

Intensification by Permanganate of Potash . — Dissolve 
quarter of an ounce of the permanganate of potash in eight 
ounces of water. When any negative which has been devel- 
oped, fixed, and washed in the ordinary way is immersed in 
tliis solution its color is changed to brown. In some cases this 
treatment alone wfill give sufficient density, and the negative 
need then be only washed and dried. 

If more density be required, wash the negative, and place it 
in the ordinary ferrous oxalate developer, when the color will 
change to black : 

KsMugOg + 4Ag3 = 4AgaO 

Permanganate of Potash and Silver prodtice Silver Oxide and 

]\1 rigOg -f- K. 0 O 

Oxide of Manganese and Potassium Oxide. 


330 


THE CHEMISTRY OF PHOTOGRAPHY. 


The first or brown image consists of a mixture of silver 
oxide and manganese oxide. 

The ferrous oxalate developer reduces the silver oxide to 
black metallic silver, which is more opaque than the silver 
oxide. 

This permanganate intensifier was used for collodion wet- 
plates as long ago as 1868, by Mr. Wharton Simpson. In 
1890 it was recommended by M. A. Grendrand, in Le Progres 
Photographique, for gelatine plates. It is better suited for 
reproductions of engravings, ^tc., than for ordinary negatives, 
as its effect is to give very strong contrasts. 

Intensifying with Aniline Dyes. — In 1890, Dr. R. E. Lie- 
segang recommended'^ the varnished negative to be coated 
with collodion or varnish in which a little of any red or green 
aniline dye had been dissolved. Such colors, it is well known, 
are bleached by exposure to light. The hack or glass side of 
the negative is then exposed to sunlight, which acts through 
the film upon the dye, decolorizing the latter in proportion to 
the opacity of the different parts of the image. 

The negative is then printed from in the usual way ; but 
the intensifying operations will have, to be repeated if many 
prints are made. 

Intensification hy the Powder Process. — By this method 
no chemical change is produced in the image ; with which, 
indeed, the materials employed do not — or need not — come in 
contact. The back or glass side of the negative is coated with 
the following mixture : 


Albumen 70 minims 

Ammonium bichromate (sat, sol.) 150 minims 

Honey 90 grains 

Water 10 ounces 


This mixture is not sensitive while wet ; but after drying 
(in a hot oven) it is much more sensitive than ordinary sensi- 
tized paper. 

The coated negative is then exposed (the film side being 
towards the sky) for about half a minute to diffused sunlight. 


* In the Photographisches Archiv. 


INTENSIFYING PEOCESSES AND THEIR CHEMISTRY. 331 

Both the exposure and the subsequent printing are best done 
at the bottom of a deep box just fitting the negative, and 
placed so that ouly parallel rajs can fall upon the negative. 

After exposure, the coated side of the negative is dusted 
over (in the dark-room) with powdered black-lead applied by 
means of a brush. This adheres to the coating in exact pro- 
portion to the action of the light upon the said coating. Thus 
a second negative is produced behind the first or original nega- 
tive; and the prints given by the double negative are, of 
course, more “intense” than those from the original film. 

By regulating the length of the exposure to sunlight, either 
the whole, or only the high-lights of the original negative can 
be intensified. 

The powder process was the invention of, or rather was 
perfected by J. Obernetter, of Munich, in 

Local Intensification . — It is often desirable to intensify a 
negative in only. It is then best to soak the negative (if 
it has been dried) in water for half an hour. Bemove the 
negative and blot off the surface water with a clean towel. 
Then paint the intensifier by the aid of a small soft brush 
upon the parts which it is desired to intensify. Wash the 
negative and complete the operation as usual. 

Another plan is to paint with machine oil upon those parts 
of the negative which do not need intensifying ; and then to 
proceed as usual. But there is some difiiculty, after the inten- 
sification is completed, in removing the oil. It may be cleaned 
off, however, by ether, or by a little weak soda. 

In the “ Photo. Almanac ” for 1889 (p. 402), G-. W. Yalen- 
tine recommends a mixture of Judson’s yellow or orange dyes 
with half an ounce of gum Senegal, applied thinly by means of 
a camel-hair brush, moistened with the tongue, to those parts 
of the negative which require intensification. 

Maxims for Intensification. 

1. ^In most intensifying processes the negatives must be thor- 
oughly fixed and thoroughly washed before intensification is 


Photographic News for 1874, pp. 147, 214, 344. 


332 


THE CHEMISTRY OF PHOTOGRAPHY. 


attempted. Sometimes intensification may fixing; and 
in one intensifying process (Edwards’) the removal of the hypo 
is of no importance. In all other cases any default in fixing 
or in washing will resnlt either in immediate failure, or in the 
appearance of spots or stains upon the film after a short time. 

2. The intensifying solutions must be kept in constant 
motion (by rocking the dish) while upon the negative. If this 
is not done, it is probable that they will act unequally npon 
the image. 

3. If a negative shows the least sign of fog^ it is better to 
slightly reduce it (see chapter on Eeducers ”) before attempt- 
ing intensification. If it is much fogged, it is useless to attempt 
intensification at all. Eeally successful intensification is only 
possible with negatives which, though thin, are quite clear in 
the shadows and full of detail. 

4. Remember that intensification is only a make-shift. It 
will generally be found better, easier, and probably cheaper to 
take another negative (when possible) rather than to intensify 
a defective one. 

.5. In choosing a method of intensification, remember that 
it mnst suit the negative. Some negatives require but a little, 
others a great deal, of intensification. Choose your process 
accordingly. 

6. Remember that all intensifiers have their good and their 
bad points. Silver intensifiers alone do not discolor the nega- 
tive; but silver nitrate stains the fingers. 

Mercurial salts, followed by baths of potassium cyanide or 
iodide and silver nitrate, etc., give permanent results; but 
they involve the use of very poisonous substances. 

Mercury, followed by ammonia, is the simplest and most 
used method ; but great care must be taken or the results are 
not permanent. 

7. Any stain upon the surface — such as the iridescent stains 
commonly seen on negatives which have been printed from 
while unvarnished — will give red spots when intensification is 
attempted. Such stains may often be removed by gentle fric- 
tion with a very dilute solution (5 grains to the ounce) of cyan- 
ide of potnssium, followed by a good washing. 


INTENSIFYING PROCESSES AND THEIR CHEMISTRY. 333 

8. If the negative to be intensified has been varnished, the 
varnish must be removed by soaking the negative in warm 
methylated spirit. It should then be washed in water and its 
surface rubbed with a pad of cotton-wool. 

Or if collodion has been used instead of varnish to protect 
the gelatine film, this substance can be removed by soaking 
the negative in a mixture of alcohol two parts, with ether one 
part. 

9. The various methods for intensification may be arranged 
in three classes : 

(a) The piling up of more silver upon the original silver 
image. 

(^) The addition of some other metal to the silver. 

(c) The substitution of some other metal, as gold or plat- 
inum, for the silver. 

10. Mercury bichloride is only slightly soluble in water, but 
much more soluble in water containing ammonium chloride. 
A speedy way of removing the bichloride is therefore to soak 
the negative (after rinsing well under the tap) for ten min- 
utes in 

Ammonium chloride 1 ounce 

Water 10 ounces 

Then rinse and wash in running water for twenty minutes. 

11. Eemember that if the values or gradations of the nega- 
tive are to be preserved, the intensifying solutions must each 
be allowed time to produce their full effect ; that is, they must 
act right through the film at every point. By arresting the 
intensification at an earlier stage the contrasts must be de- 
creased, because the solutions will have penetrated completely 
through the detail in the shadows, while in the high-lights 
their work may be less than half done. 


CHAPTER XXX. 


THE TONING OF PHOTOGRAPHS CONSIDERED 
CHEMICALLY, HISTORICALLY, AND 
GENERALLY. 

Fizeau Discoyeks How to Tone Daguekeeotypes. 

What Is “Toning’’? — The term “toning” is used in pho- 
tography in the sense of “ coloring.” In fact, the early writers 
on photography actually used the word “ coloring,” and not 
“ toning,” for tlie process which we are about to describe. 

When an ordinary photograph on paper is taken out of the 
printing-frame, its color may be pleasant or unpleasant to the 
eye. With the greater part of the ready-sensitized papers 
now sold the color is distinctly unpleasant, being a red of un- 
certain tint. With freshly sensitized paper the color approxi- 
mates more to violet. But all such prints, if fixed at once 
without toning, assume a brick-red hue which is inartistic and 
displeasing to the eye. 

To change this red color to brown or black is the object of 
the ]fiiotographic process known as “ toning.” It is effected, 
for the most part, by depositing finely divided gold upon the 
silver which forms the picture. 

Faraday has shown* that gold in an extremely fine state of 
division may be of many colors, from ruby to blue. It is the 
blue form of gold which we desire to deposit upon the red 
silver of the print in order to “tone” it. The combined 
effect of the red and the blue is to give the blackish tints 
which we desire. 

The Toning of Daguerreotypes. — The first successful 

* “Some Observations on Divided Gold Proceedings Royai Institution^ Volume II., 
for 1854-58 ; pp. 308-312. 

“ On the Relations of Gold to Light:” Proceedings Royal Institution^ pp. 444-46. 

“Experimental Relations of Gold (and other metals) to Light” (Bakerian Lecture): 
Philosophical Transactions for 1857, pp. 14.5-162 ; also in P hilosophical Magazine for 1857, 
pp. 401-512. 


THE TONING OF PHOTOGRAPHS, ETC. 335 

photograpliic process was that which bears the honored name 
of Daguerre, and which he published in 1839. The image 
was formed (in the camera) upon a silver plate covered with 
iodide of silver, and it was developed by the vapor of mer- 
cury. The early daguerreotypes were very weak, indistinct, 
and unstable productions ; and the toning or gilding of them 
by means of a solution of chloride of gold, as discovered by 
the French investigator, M. Hippolyte Louis Fizeau, in ISdl,* 
was a great improvement. 

After development, the silver plate had its picture fixed ” 
by immersion in a solution of hyposulphite of soda. The fol- 
lowing description of its subsequent treatment is extracted 
from M. Fizeaii’s memoir : 

Since the publication of the photogenic processes every 
one, and M. Daguerre among the first, acknowledged that 
something yet remained to be done to give these marvellous 
images that degree of perfection which it is now possible to 
obtain : I mean the fixing of the impressions and the giving 
to the light parts of the image more intensity. 

The process which I now submit to the Academy appears 
to me to resolve in a great measure this double problem ; it 
consists in subjecting the plate to tlie action of a salt of gold 
prepared in the following manner : 

“ Dissolve 1 gramme of cldoride of gold in 1 pint of pure 
water, and 3 grammes of hyposulphite of soda in another 
pint of water ; then pour the solution of gold into that of 
soda, little by little, and. shaking it all the while. The 
mixture, which is at first of a slightly yellow color, soon 
becomes perfectly limpid. It would then appear to contain a 
double hyposulphite of soda and of gold, with the addition of 
marine salt, which appears to perform no active part in the 
operation. 

“ In order that this salt-of-gold process may produce its 
effect upon the silver coating of the plate, it is important that 
the latter should be perfectly free from foreign matter, and 
especially from all greasy particles ; it is therefore necessary 


* “ Sur un moyen de fixer les images photographiques ” : Paris. Comptes Rendus, 
Volume XI,, pp. 237-8. 


336 


THE CHEMISTRY OF PHOTOGRAPHY. 


that it should have been previously washed with great care, 
which may be dispensed with when you only wish to have 
recourse to tlie ordinary wash. 

The following method is the one most generally attended 
with success : While the plate is yet covered with the coating 
of iodine, but exempt from all dust and grease, both on the 
two surfaces and at the edges, pour a few drops of alcohol 
upon the iodized surface. 

When the alcohol has wetted the whole surface, immerse 
the plate first in the filtered w^ater, and afterward in the hypo- 
sulphite solution. This last must be renewed for each plate, 
and should contain about 1 part of salt of gold to 15 of water; 
the remaining part of this washing process is performed in the 
ordinary way, only care should be taken that the water used 
should be as much as possible free from dust. 

“ The alcohol is used simply to cause the water to adhere 
perfectly to the whole of the surface of the plate, and to 
hinder it from running off to the sides on each immersion, 
which would infallibly cause spots. 

When a plate has been washed with these precautions, 
even if the image w’as very old, the application of the salt of 
gold would be the most simple possible. You have only to 
place the plate upon the wire frame, which is to be found in 
each apparatus ; to pour upon it a coating of the salt of gold, 
sufiicient to cover it entirely, and to heat it underneath with a 
strong flame. The impression will be found to become dis- 
tinct, and to assume, in a minute or two, a fine vigorous tone 
and color. When the effect is produced, the liquid must be 
poured off and the plate washed and dried. 

‘‘ In the operation which we have just described, the follow- 
ing phenomena have taken place : Silver has been dissolved, 
and gold has been precipitated upon the silver, and also upon 
the mercury, but with very different results. The silver 
which, by its polish, forms the dark part of the picture, is in 
some degree browned by the thin coating of gold which covers 
it, whence results an increased intensity in the black parts’; 
the mercury, on the contrary, which, under the form of 
infinitely small globules, forms the whites, increases in 


337 


THE TONINH OF PHOTOGRAPHS, ETC. 

strength and brilliancy by its amalgamation with the gold, 
whence result a greater degree of fixity and a remarkable 
augmentation in the light parts of the image.” 

Before Fizeau announced this method of gilding or toning, 
we are told that “ the daguerreotype would not resist the 
slightest touch ; a finger passed over it destroyed the wdiole 
picture ; moreover, it did not long remain intact — a short time 
sufficed to deprive it of its sharpness.” 

This paper of Fizeau’s was the origin of our system of 
toning, and, as such, marks an epoch. We note : 

1. That it introduces the use of the chloride of gold into 
photographic processes. 

2. The gold chloride was not used alone, but combined with 
hyposulphite of soda. 

3. followed fixing. 

4. The deposition of the gold upon the image was produced 
by the agency of heat. 

5. The permanency and the color of the image were both 
improved. Grold is a far less oxidizable metal than silver ; so 
that it stands exposure to the air without material alteration, 
while silver rapidly tarnishes. Its color, too, is superior to 
that of the silver alone. 

The practice of the daguerreotype process has ceased. It 
ended in England about 1855, and in the United States about 
1863. But it was the first practical and commercial process 
of photography, and it is interesting to trace its infiuence upon 
the processes which have superseded it. 

History of Toning Processes. 

Hardwick upon Toning . — In March, 1855, T. F. Hardwich 
published the first edition of a book which became so well 
known among English workers that it was dubbed the “ Pho- 
tographer’s Bible.” The author was an excellent chemist, and 
he did much in those early days to put the scientific side of 
photography upon a sound footing. His book has since passed 
through nine editions, having been revised by Messrs. Dawson, 
Hadow and Traill Taylor, the date of the latest edition being 
1883 ; and it is still a sound and useful work. The long 


338 


THE CHEMISTRY OF PHOTOGRAPHY. 


period of time (nearly thirty years) over which the issue of 
the nine editions extends, and the changes necessarily made in 
each edition in accordance with new discoveries, cause the 
study of this volume to have an important bearing upon the 
history of photography. 

In the hrst edition the word coloring” is used instead of 
“ toning,” and it is pointed out that when a bath of old 
hypo ” is used, the dark tints produced are due to the com- 
bination of sulphur with the silver of the print, forming sul- 
phide of silver. 

It is also carefully pointed out by Hardwich that, when 
chloride of gold is added to hypo, not only is the double salt 
called “seld’or” produced, but also tetrathionate of soda, 
which latter salt is readily decomposed, liberating sulphur. 
Thus a newly made ‘‘ coloring ” bath of this kind does, it is 
true, color prints by a deposition of gold, but old baths effect 
the work mainly by means of sulphur. He adds that crystal- 
lized sel d^or (which is a double hyposulphite of gold and soda 
freed from tetrathionate) can be used for the coloring bath.* 

In the second edition of Hardwich (September, 1855) the 
word ‘‘ toning ” replaces “ coloring.” The sulphur toning 
bath of old, or acid hypo, is still the first one mentioned ; Le 
Gray’s method is next given ; then that of toning and fixing in 
one bath containing hypo and gold ; and, lastly, a new method 
by Thomas Sutton, in which crystallized sel d’or is dissolved in 
water acidified with hydrochloric acid. 

In the third edition of Harwich, June, 1856, sulphur toning 
is condemned and Sutton’s acid sel d’or bath recommended for 
toning ; but there is an important line which shows the birth 
of a new epoch in toning : ‘‘ M. Le Gray’s process is objec- 

tionable on account of the excessive overprinting required. 
This, however, is to a great extent obviated by a modification 
which the writer has seen, where an cdhaline instead of an acid 
solution of the (gold) chloride is employed.” 

The fourth edition (April, 1851) still lays stress on the sel 
d’or toning bath, and it contains the first description, in print, 

* Sel d’or was discovered by M.M. Fordos and Gelis in 1843, and was known com- 
mercially as “Gelis’s salt.” — W. J. H. 


THE TONING OF PHOTOGRAPHS, ETC. 


339 


of an alkoMne gold toning bath. This bath was the discovery 
of Mr. W aterhouse, and will be described more fully later on. 

Alkaline toning comes strongly to the front in the fifth edi- 
tion of Hardwich (1859), and a formula endorsing the use of 
bicarbonate of soda is given, but the sel d’or bath is still strong 
in the running. 

The last edition of his book brought out personally by Hard- 
wich was the sixth (1861), in which there is little change ; but 
in the seventh (1861), edited by Dawson and Hadow, the alka- 
line toning bath takes a strong lead, though we are told that 
the method of fixing and? toning in one bath is even now 
sometimes follow^ed ” (p. 306). The two later editions add 
nothing of importance. 

Toning by Other Metals than Gold. 

It has been proposed to use such metals as palladium, iridium, 
etc., as toning agents ; and experiments have shown that pleas- 
ing tints can be obtained by their use, but for various reasons — 
chiefiy the enhanced expense— none of them have come into use. 

But with one metal — platinum—it is possible that the case 
may be different. 

The use oi i)latinum as a toning agent was first proposed by 
M. Caranza in the French journal, La L%imiere^ for February, 
1856. A Scotchman — Burnett — experimented in the same 
direction in the years 1858 and 1859 ; and quite recently (1888) 
Mr. Lyonel Clark has obtained considerable success by the use 
of this noble ” metal. 

Toning with Platinum. 

The only metal which is likely to compete successfully with 
gold is platinum. The best salt of platinum by far for toning 
purposes is the chloro-platinite of potassium,” K 2 PtC ]4 (not 
the ordinary chloride or bichloride of platinum, PtCl 4 ). Make 
up a stock solution of 60 grains of this salt to 2 fluid ounces 
of distilled water. For the regular toning bath use : 


Stock solution of chloro-platinite 1 fluid drachm 

Nitric acid 2 drops 

Water Bounces 


340 


THE CHEMISTRY OF PHOTOGRAPHY. 


The prints are to be well washed, and are afterwards 
immersed or floated upon the toning solution. When the 
desired tone is obtained (and this bath yields tints from brown 
to black) they should be removed, washed in alkaline water, 
and fixed in hypo as usual. This method is due to Mr, Lyonel 
Clark ; it answers better with matt-surface than with albu- 
menized paper. By using the bath stronger (only 2 ounces 
of water instead of 8), much blacker tones are obtained. 

Classification of Toning Processes. 


We are now in a position to enumerate the various toning 
processes which have been practised since the discovery of 
photography. Having done this, we shall consider, in turn, 
the chemical changes upon which each process depends. 

( 1. By old hypo. 

1.— Sulphur toning- 

I 2. By acid hypo. 


II. — Gold toning. . 


fi. 

-ig; 

U- 


By mixture of hypo and gold chloride. 
By sel d’or. 

By acid gold chloride. 

By gold chloride plus an alkali. 


III. — Platinum toning. 

IV. — Toning by other metals. 


Alkaline Toning with Carbonates and with Borax. 

Alkaline Tonmg with Chloride of Gold Originated ly 
Waterhouse in 1855.— In the early days of photography- 
thirty or forty years ago — the Photographic Society of Lon- 
don did good service in appointing several committees to con- 
sider such questions as the causes of fading of prints, etc. 
One of the most active members of that day was Mr. T. F. 
Hardwich, and the first indication which we get of the use of 
an alkaline toning bath is contained in a short paper On 
Gold Toning Applied to Albumenized Paper,” published in 
the Photographic Journal for December 11, 1858 (p. 95). 
In this paper Hardwich refers to ‘'the labors of the Printing 
Committee appointed by your society.” It had been said that 
these labors “issued in nothing, and that they found all their 
pictures to fade.” He then protests against this statement,, 
remarking : 


THE TONING OF PHOTOGKAPHS, ETC. 


311 


“ I have in my hands cards on which the prints experimented on by the 
Priming Committee are mounted ; and these cards show that although 
many pictures have not proved permanent, yet that others, printed in a 
different way, have stood severe tests. 

“In examining these cards, we may take, for instance, the proofs 
toned by sel d’or, contributed by Mr. Shadbolt. Three months' suspen- 
sion in air saturated with water has made no impression on them, and 
although they have been mounted more than two years since that time, 
they are still unaltered. 

“ Or, again, let us examine the condition of certain prints sent to the 
committee by Mr. Waterhouse, of Halifax. I have mounted one of them 
to show you that no perceptible difference can be made out between the 
two halves, although one has been subjected to the ordeal above men- 
tioned. 

“It is with reference to Mr. Waterhouse’s process that I wish to 
address you this evening ; and since it appears likely to become very 
popular, it may not be without interest if I describe briefly how it origin- 
ated. The prints were sent to the committee by Mr. Waterhouse with the 
following letter, as far as my memory serves me : ‘ I salt the paper with a 
chloride dissolved i)i a solution of caseine, 07id tone the image with chloride of 
gold. But inasmtich as Le Gray' s p7'ocess eats into the picture^ I modify it by 
using an instead of an acid solution of gold. The alkali I employ 

is the potassce subcarb., and I add more or less of it accordittg to the titit 
desired."' " 

Tlie ‘‘potassse subcarb.” of the druggist is our carbonate 
of potash. 

The Printing Committee ’’ referred to by Hardwich in 
the paper from which the above remarks are quoted was 
appointed at the meeting of the Photographic Society of 
London, on 3d of May, 1855 ; and its first ” (and apparently 
only) report is printed in the Photographic Journal for 21st 
of November, 1855. Mr. Waterhouse’s prints, with his 
remarks as quoted by Hardwich, must therefore have been 
sent to that committee about the middle of the year 1855, and 
this is the period from which ‘‘ alkaline ” gold toning dates. 

The Modern Carbonate of Potash Bath . — The carbonate of 
potash bath gives lovely warm or sepia tones. It may be 
made up as follows : 


Chloride of gold 1 grain 

Carbonate of potash 15 grains 

Distilled water 10 ounces 


The bath should be made up an hour or so before using. 


342 


THE CHEMISTRY OF PHOTOGRAPHY. 


It keeps fairly weil. Dissolve the potash in the water, and 
add the gold last of all. The best temperature for the bath is 
65 deg. Fahr. 

The Carbonate of Soda Bath — Hardwich^ 185Y. — In the 
fourth edition of his well-known Manual of Photographic 
Chemistry,” published in 1857, T. F. Hardwich makes the 
first mention in print of an alkaline gold toning bath. He 
writes (p. 132) : 

“ M. Le Gray’s toning process (using nothing but acid chloride of 
gold) is objectionable on account of the excessive over-printing required. 
This, however, is to a great extent -obviated by a modification of the proc- 
ess in which an alkaline instead of an acid solution of the chloride is 
employed ; 1 grain of chloride of gold is dissolved in about 6 ounces of 
water, to which are added 20 to 30 grains of the common carbonate of 
soda. The alkali moderates the violence of the action, so that the print, 
washed with water and immersed in the gold bath, is less reduced in 
intensity, and does not acquire the same inky blueness. On subsequent 
fixing in the hyposulphite, the tint changes from violet to a dark chocolate 

brown, which is permanent.” 

• 

For the important idea of an aTkaline gold toning bath, 
Hardwich was indebted to Mr, Waterhouse, of Halifax, the 
inventor of the generally used ^‘Waterhouse diaphragms.” 
This is clear from a statement contained in a paper by Hard- 
wich in the Photographic Journal for 11th of December, 
1858 (p, 95). In this he states that Mr. Waterhouse sent to 
the “Printing Committee” a set of prints toned by an alka- 
line solution of gold ; the alkali used being carbonate of pot- 
ash. Hardwich then writes : “ Finding that this process was 
more manageable than Le Gray’s, and produced very perman- 
ent pictures, I was induced, in an edition of the ‘ Manual of 
Photographic Chemistry,’ which appeared about that time, 
to suggest a trial of it, having previously adjusted the propor- 
tions, and substituted carbonate of soda for carbonate of pot- 
ash, as a salt more easily obtainable.” It was unfortunate that 
no mention of Waterhouse’s name was made in the fourth 
edition of Harwich’s book in 1857, but the omission was cer- 
tainly not intentional, for the author was one of the most open 
and honorable of men. , 


* The fourth edition, 1857 ; see p. 132. 


THE TONING OF PHOTOGEAPHS, ETC. 

The Modern Carbonate of Soda Toning Bath. 


343 


Chloride of gold 1 grain 

Carbonate of soda (sal soda) 12 grains 

Distilled water 10 ounces 


The solution should be made up half an hour before it is to 
be used. With strong, intense negatives, possessing numer- 
ous gradations, this bath gives a rich purple-black tone. 

The Borax Toning Bath. — Writing of the phosphate of 
soda bath in 1859,* Mr. Maxwell Ljte remarks : ‘‘180 grains 
of borax may be substituted for phosphate of soda with a like 
result.” A borax bath containing a little common salt is also 
described by Mr. John Hey wood, in the British Journal of 
Photography for the same year (p. 282). 

The Modern Borax Bath. 

Borax 60 grains 

Chloride of gold 1 grain 

Distilled water 8 ounces 

Warm the water to about 100 deg. Fahr., dissolve the pow- 
dered borax in it, and then add the gold. Allow to cool to 
70 deg. Fahr. before using. 

The borax bath is ready for use immediately it is made up ; 
but it does not keep well, and it is preferable to only make up 
as much solution as will tone the prints in hand. It seems to 
agree specially well with the ready-sensitized papers now so 
largely used. 

Brooh's Borax Bcdh. — Wash the prints well in plain 
water. 

Dissolve 90 grains of powdered borax in 15 ounces of hot 
water. When cooled down to, say, 75 deg. Fahr., add 1 grain 
of chloride of gold and shake well. This ought to tone one 
sheet of paper. Keep the prints moving. The borax solution 
must freshly made; a stock solution of borax does not 
answer nearly so well.f 

Chemical Changes in the Borax Bath. — It is not easy to 
follow positively the chemical changes which accompany the 


* Photographic News^ p, 301. 

tThis agrees with my own experience.— W, J. H. 


344: 


THE CHEMISTEY OF PHOTOGrEAPHY. 


toning of a silver print in the borax bath,” but the follow- 
ing equation represents what probably takes place : 


3Na2B407 

+ 

ISHgO 

+ 2 AUCI 3 

= I 2 H 3 BO 3 

+ 

Borax 

and 

Water 

and Gold 

produce 

Boric 

and 




Chloride 

Acid 





NaClOg + 

5NaCi 

+ 

2Au 




Sodium and 

Sodium 

and 

Gold. 




Chlorate 

Chloride 




The sodium chlorate, which is one of the substances pro- 
duced during the above reaction, attacks the silver sub- 
chloride (the ‘^reduction product” produced by the action 
of light on silver chloride) and weakens the print somewhat. 
Hence the necessity of over-printing to some extent when the 
borax bath is to be used. 

Burnett adds Common Salt to the Alkaline Toning Bath 
{1859). — Mr. C. J. Burnett contributed to the British Journal 
of Photography for 1859 (Yol. YI., p. 175), a formula for a 
carbonate of soda gold-toning bath, to which he recommended 
the addition of ‘^common salt, 5 to 10 grains per ounce.” 
The reason of this he stated to be that ‘^chloride of sodium 
(common salt) prevents precipitation of gold even when kept 
long.” The addition of a little salt has since been recom- 
mended in the formulas of several workers, and doubtless with 
the same ideas in view — that it makes the bath keep better. 
Its action in this direction is doubtless due to the affinity of 
sodium chloride for the sodium chloro-aurate which constitutes 
the active ingredient of most toning baths 

The Acetate Bath. 

The Acetate Toning Bath of Ilannaford and Labor de^ 1859. 
— The first mention which we have met with of the use of 
acetate of soda in the toning bath occurs in the report* of a 
meeting of the South London Photographic Society, held 
more than thirty years ago, when, during the discussion of a 
paper on Positive Printing,” Mr. Ilannaford said that 
recently he had employed acetate of soda with the gold.” 

But the first published formula for the use of the acetate 

^■Photographic Journal for November 15, 1859, p. 83. 


345 


THE TONING OF PHOTOGRAPHS, ETC. 

bath appears to be that of the Abbe Laborde, which was given 
in the British Journal of Photography for August 15, 1860 
fp. 240). 

“Dissolve in water 35 ounces 

Acetate of soda drachms 

Chloride of gold 15 grains 

“The solution becomes colorless by degrees, and at the expiration of 
twenty-four hours it is ready for use. 

“ If the gold bath has been used before, its action will be slower.” 

By the allusion to the bath having been “ used before,” it 
appears that Laborde was acquainted with what is perhaps the 
most valuable property of the acetate toning bath — its keeping 
qualities. Most toning baths require using the same day that 
they are made up ; but with proper care the acetate bath will 
last for years. Hence it is especially useful to that class — a 
large one among amateurs — who tone only a few prints at a 
time. 

Laborde’s formula would now be considered too strong ; and 
the following may be considered as the accepted formula of 
the present day for the acetate toning bath : 

Modern Acetate Bath. 


Distilled water 8 ounces 

Chloride of gold 1 grain 

Acetate of soda 30 grains 


Dissolve the acetate in the water at a temperature of 80 or 
90 deg. Fahr.; add the chloride of gold to it ; and use it when 
the temperature has sunk to about 65 or 70 deg. 

If the gold chloride has not been previously neutralized, it 
is a good plan to add a pinch of powdered chalk to the acetate 
bath ; it removes any free acid which may be present. 

A Preliminary Bath in Salt and Water to Pemove Free 
Nitrate. — The acetate bath is one which requires the free 
nitrate of silver, and indeed all the soluble salts of silver, to be 
removed before the print is immersed in the toning-bath. This 
is best done by rinsing the prints in three changes of water 
and then soaking them for five minutes in water to which 
common salt has been added in the proportion of a teaspoonful 


346 


THE CHEMISTRY OF PHOTOGRAPHY. 


to every quart. This will redden the prints considerably ; and 
the reddening is in itself a good thing, as it makes the subse- 
quent changes of color more perceptible. 

This salt-water bath slows the subsequent toning, and if too 
much salt be added toning will be rendered difficult. 

After soaking in the salt-water for five minutes the prints 
should be again twice rinsed in plain water, when they are 
ready to be toned. 

The acetate bath is preferred by those who like warm ” 
tones, by which is meant shades of rich brown with a tinge of 
red in them. 

Keej[>ing Powers of the Acetate Bath. — The acetate bath is 
a favorite with amateurs, because it can be kept ready mixed 
and used over and over again. Like most alkaline or neutral 
solutions of gold, it is affected by light, so that the bottle con- 
taining it should have two or three thicknesses of brown paper 
pasted round it and should be kept in a dark cool corner. 

But many people make the mistake of wanting the toning 
bath to do too much, expecting it to tone after its gold has 
been exhausted. If w'e reckon that a grain of gold will tone 
a sheet of paper, we see that the limit of the toning power of 
the modern acetate bath, made up according to the formula 
given above, would be six whole-plate prints, or twenty-four 
quarter-plates. But if we make up a good supply of the solu- 
tion — say a quart — and add as much gold solution and water 
every time after %ising as will make the solution up to its 
original measure, calculating the amount of gold to add by 
reckoning the number of prints toned, then there is no reason 
why, with proper treatment, the acetal e bath should not last 
indefinitely. The water added should also contain soda acetate 
dissolved in the proper proportion. Many workers can 
point to acetate baths which they have had in use “ for years,” 
although it is more than probable that not a drop of the 
original bath, owing to the repeated renewals, remains, 

Barnes’ Acetate Bath.~T\\^ following is a good method of 
working the acetate bath, and is the formula of Mr. C. B. 
Barnes 


* British Journal of Photography for 1889 ; p. 96. 


THE TONING OF PHOTOGRAPHS, ETC. 


347 


“ Into a gallon stone jar break a fifteen-grain tube of chloride of gold, 
and add half an ounce of acetate of soda, and a small pinch of chloride of 
sodium (common salt) ; pour on this about a pint of boiling water and let 
it stand for an hour or so, then fill up the jar with rain or distilled water, 
and let it stand for at least twenty-four hours before using. When the bath 
is required for toning, pour into the dish just the quantity required for 
present use, and when the toning is completed throw the used solution 
away. That in the jar will keep good for years, and as no used-up or 
partially used-up solution is poured back, it can be used to the last drop 
without requiring the addition of fresh gold ; added to which it cannot 
become contaminated by anything which might find its way into the toning 
dish, or that portion which has been used therein,” 

To this we would add the caution — be sure that your gallon ” 
stone jar is well glazed within, and scrupulously clean. 

The acetate bath was the favorite toning bath of M. Adam 
Salomon, the famous French sculptor-photographer, whose 
work was so much admired in the “ sixties.” If a fresh 
acetate bath works slowly or with difficulty, it is a good plan 
to give it a start by adding two or three grains of bicarbonate 
of soda. 

Acetate Bath Ready for Immediate Use, — Put 2 ounces of 
acetate of soda into an earthen jar, and break in the same jar 
a 15-grain tube of chloride of gold. Pour a pint of boiling 
water over the mixture, and stir well with a glass rod. Allow 
the liquid to stand for a quarter of an hour (shaking up 
occasionally), and then pour it into a larger vessel containing 
five pints of cold water. Stir well, and the bath is ready for 
use. 

At first this bath will work very quickly, and the prints will 
reach the slaty-blue tint which marks the over-toned ” stage 
in three or four minutes. Take them out early, and you will 
get rich deep sepia tones. This bath is given by Mr. Geo. 
Bradforde in the Photo News Year-Book for 1881 (p. 108). 

Chemical Action of the Acetate Bath. — According to 
Abney, the chemical changes which take place in the acetate 
toning bath may be expressed by the following equation : 

2 AUCI 3 + NaHgCgOg =: 2Au -f 

Gold chloride and Sodium acetate produce Gold and 
NaClg CgOg -h 8HC1. 

Sodium Trichlor-acetate arid Hydrochloric acid. 


348 


THE CHEMISTRY OF PHOTOGRAPHY. 


The acetate thus combining with the free chlorine liberated 
from the gold chloride, and thereby preventing it from attack- 
ing the silver sub-chloride which forms the dark parts of the 
print. 

The Phosphate Bath and Lime Baths. 

The Phosphate Toning Bath of Maxwell Lyte (1859). — In 
a, communication* to the Photographic Society of France, Mr. 
Maxwell Lyte, a well-known English amateur then residing in 
France, gave the following instructions for toning prints : 

“ Over-print a little. Wash, first in plain and th^m in salt-water, for ten 
minutes. Make up the following toning bath : 

(Original Phosphate Bath of 1859.) 

Chloride of gold 15 grains 

Phosphate of soda (the purified tri-basic phos- 
phate of commerce) 300 grains 

Distilled water 1^ pints 

“This bath ought to be completely neutral, or at all events rather alka- 
line than acid.” 

Here, again, we should consider this bath as too strong in 
gold. The “phosphate bath” now generally used is made 
up as follows : 

Modern Phosphate Bath. 


Chloride of gold 1 grain 

Phosphate of soda 20 grains 

Distilled water 8 ounces 


The tones given by this bath are of a rich purple ; but the 
toning should be carried slightly beyond this, or until the 
prints are of a full violet or violet- black hue, as they “go 
back” somewhat during the subsequent processes of fixing 
and washing. 

Tliis phosphate toning-bath will keep for some little time 
before using, and indeed is better if made up an hour before 
it is required ; but it cannot be used a second time, so that no 
more should be mixed than is required. As in all toning 
baths, the best plan is to dissolve the soda in the water, and 


* Reprinted in Photographic News for March 4, 1859, p. 301. 


THE TONIKG OF PHOTOGKAPHS, ETC. 


349 


add the gold last of all. The bath should be quite colorless 
before it is used ; but it ought to lose its yellow hue (caused 
by the addition of the gold salt) in a few minutes. 

It will be noticed that Maxwell Lyte recommended bathing 
the prints in salt-water before placing them in the phosphate 
bath. This practice is not now, how^ever, generally followed. 
Indeed, Abney recommends that with this ‘‘toner’’ a little 
free silver nitrate be left in the print. It is usually enough to 
rinse the prints in three changes of water— rapidly in the first 
one, and allowing two or three minutes only in each of the 
others — to have the prints in the best possible condition for 
toning in this phosphate bath. 

Cause of “ Measles ” in Silver Prints . — Sometimes the 
prints, after they have been toned and fixed, show a number 
of small white and red specks all over their surface, producing 
what professional printers have termed ‘* mealiness ” or “ meas- 
liness ” in the prints. The cause of this is that little or no 
free nitrate of silver has been left in the sensitized paper. 
Perhaps the sensitizing bath was too weak in silver, or the 
paper may have been washed after sensitizing in order to make 
it keep better. When such paper is exposed to light (as it 
must be during printing) the silver chloride is decomposed into 
black silver sub-chloride and chlorine : 

2AgCl = AggCl + Cl 

Silver Chloride produces Silver Sub-Chloride and Chlorine. 

The free chlorine attacks the albuminate of silver, and com- 
bines with some of its silver to form little spots of fresh silver 
chloride, which, being then acted on by light, is blackened in 
its turn, but to a slightly different tint. It is these spots or 
specks which produce the “ measles.” 

The best remedy for measles is to fume the paper for ten 
minutes before printing. This is usually done by exposing 
the paper in a closed box having a perforated false bottom 
(underneath which is a saucer containing a little strong am- 
monia) to the fumes or vapor of ammonia ; or the ;pads of the 
printing-frame may be fumed instead of the paper. The am- 
monia then combines with the clilorine as fast as the latter is 


SoO 


THE CHEMISTEY OF PHOTOGRAPHY. 


liberated, and ammonium chloride is formed, which is a quite 
harmless substance : 

4 NH 3 + 3C1 = 3 NH 4 CI + N 

Ammonia and Chlorine d’f'odnce Ammonium Chloride and Nitrogen. 

Toning with Salts of Lime, — Three of the salts of lime 
have been and are commonly employed in the processes of 
toning. 

The carbonate of lime (CaCOg) is usually employed in the 
form of powdered or “ precipitated ” chalk to neutralize the 
hydrochloric acid which is invariably present in commercial 
chloride of gold. 

The true ‘‘chloride of lime,” or calcium chloride (CaCl^), 
is employed in certain toning baths. 

Commercial “ chloride of lime,” or chlorinetted lime (often 
called “ bleaching powder,” and also much used for disinfect- 
ing purposes), is a mixture of calcium chloride (CaClg) and 
calcium hypochlorite (CaClgO). 

Le Gray Introduces the “ Chloride of Lime ” Toning Bath, 
— Gustave Le Gray, the famous French photographer of forty 
years ago, was a man not unwilling to recognize improvements, 
even in his own discoveries. His introduction of acid chloride 
of gold as a toning bath about 1850 having been objected to 
on account of the great amount of over-printing necessary, 
and Waterhouse and Hardwich having shown in England 
(1855-8) that an alkaline solution of gold was preferable, Le 
Gray announced to the French Photographic Society early in 
1859 that ordinary bleaching powder (the commercial “ chlo- 
ride of lime”), added to a Solution of chloride of gold, made 
a toning bath far superior to his former acid bath. His 
formula* was : 


Distilled water 1000 parts 

Commercial chloride of lime 1 part 

Chloride of gold 1 part 

Chloride of sodium 1 part 


This bath tones slowly but regularly, and gives black tones. 


* Reprinted from the French Bulletin in Sutton’s Photographic Notes for 1859, pp. 
41, 106. 


THE TONING OF PHOTOGEAPHS, ETC, 


351 


Sutton’s Lime Bath. — In Button’s pamplilet on “ Positive 
Printing’^ (1863) he writes : “ The best toning bath, and that 
which I most strongly recommend, is a solution of a double 
salt of gold, called ^ calcio chloride,’ which consists of a com- 
bination of chloride of gold with chloride of calcium, ren- 
dered slightly alkaliue by an excess of chloride of lime. 
This solution is as limpid and colorless as water, does not 
become decomposed by keeping, and is always ready for use.” 

Lime Bath with Chalk. — Shake ujd 40 grains of powdered 
chalk with 1 pint of hot distilled water. Add 3 grains of 
chloride of lime and shake again. Lastly, add 2 grains of 
chloride of gold. Shake a third time and allow to stand till 
cool (65 deg, Fahr.) ; the bath is then ready for use, though it 
will work much better after keeping for a day. This bath 
gives black tones with good negatives, and paper which is not 
too old. 

A Modern Lime Bath. — Make up three stock solutions : (A) 
15 grains of gold chloride in 7^ ounces of water ; (B) J pound 
of slaked lime (calcium hydrate, CallgO^) in a quart of water; 
shake well and allow to stand till the excess of lime has sunk 
to the bottom ; (C) 1 ounce of dry calcium chloride dissolved 
in 1 quart of water. To make up the toning bath, take \ 
ounce of the chloride of gold solution and shake it up in 3 
ounces of water ; add to this the B solution (lime-water) until 
the color of a bit of red litmus paper placed in it is just 
changed to blue ; then add \ ounce of the C solution, shake 
well, and the bath is ready for use. 

The Bicarbonate Toning Bath. — Bicarbonate of soda was 
used in a complicated ‘Toning and fixing” bath by M. Jobard,"^ 
in 1859. In November of the same year Mr. John Hey wood 
gave a formula in the British Journal of Photograjjhy, p. 282, 
in which he recommends the prints to be well washed in both 
plain water and salt water, and then a bicarbonate toning solu- 
tion to be laid on with a brush in a manner which we shall 
describe further on. 

In 1863 Mr. Gr. Spiller gave the following formula for a 

* See Bulletin de la Socie'te Francaise de Photographie for 1859 j translated in Photo- 
graphic Journal for same year, p. 8. 


352 


THE CHEMISTRY OF PHOTOGRAPHY. 


bicarbonate bath in a paper which he read* before the Photo- 
graphic Society of London : 


Chloride of gold 5 grains 

Bicarbonate of soda 20 grains 

Water 1 pint 


All these old toning baths err in being too strong in gold. 
The Modern Bicarbonate Bath. 


Chloride of gold 1 grain 

Bicarbonate of soda 5 grains 

Distilled water 10 ounces 


This bath is ready for use ten minntes after it is made ; but 
it will not keep. 

The Tungstate Bath . — A formula given by Mr. A. Hughes, 
in 1865, f reads : “ Take the chloride of gold and just neutralize 
with tungstate of soda, and then to each grain of gold add 
20 grains of the tungstate ; dilute with boiling distilled water, 
and when cool the bath is ready for use. Distilled water is 
mentioned, as common waters vary so much that they some- 
times upset all formulas. In strengthening this toning bath, 
the gold may be simply neutralized with the tungstate, the 
excess not being required. This bath can be kept and 
strengthened from day to day, as required, ad infinitum. 
It is found to tone to a rich purple one and a quarter sheets of 
paper with 1 grain of chlori(Je of gold.” 

Modern Tungstate Bath. 


Tungstate of soda 20 grains 

Chloride of gold 1 grain 

Boiling water 8 ounces 


Heady for use , as soon as cold. Add more gold, with a 
grain or two of tungstate, at the end of each day’s work. 

Carbonate of Magnesia Toning Bath . — In April, 1866, Mr. 
E. Seeley read an account of a gold toning bath containing 
carbonate of magnesia before the Horth London Photographic 
Association,:}; in which he emphasized the following points : 


* Photographic Journal^ vol. viii., p. 410. 
t British Journal of Photography^ p. 206. 
:j: Photographic News^ 1866, p. 173. 


THE TONING OF PHOTOGRAPHS, ETC. 


353 



The gold chloride should first be neutralized (as sold it is 
always acid) by the addition of a little carbonate of soda or 
powdered chalk. 

The carbonate of magnesia should be well shaken up with 
warm (80 deg. Fahr.) distilled water. In this it is only slightly 
soluble, 50 ounces of water dissolving only 1 grain. The solu- 
tion is then alkaline to test-paper. Let the solution stand and 
then pour otf the clear part. Add 1 grain of gold chloride to 
every 20 ounces of the clear solution. The toning bath so 
prepared is ready for use after twenty-four hours. It will 
keep well for several days, but slowly loses its power after that. 
When used it should always be slightly alkaline to test-paper. 

When using this ^hnagnesia” bath the prints should not have 
all the free nitrate of silver washed out of them. The last 
wash-water used should be decidedly milky. 

This bath requires a well-silvered paper, and would there- 
fore be of little use with much of the ready-sensitized cheap 
paper of the present day, most of which is sensitized by floating 
on a bath containing only about 30 grains of silver nitrate to 
the ounce of water. The best proportion is double this amount, 
or from 50 to 60 grains to the ounce. 

Seeley claimed that with the carbonate of magnesia bath at 
least five sheets of paper of the full size can be toned with one 
grain of the chloride of gold. W^e usually tone six and some- 
times seven.” It gives black tones. 

The Benzoate Bath . — In 1864 Mr. Carey Lea described a 
benzoate of potash toning bath in the PhiladelpJda Photog- 
rapher^ which he considered gave even better tones than the 
acetate bath, lie writes: “Three or four grains of caustic 
potash are dissolved in water in a glass vessel, and the solution 
is supersaturated with benzoic acid. The exact quantity of 
the acid is unimportant, provided that rather more than 
enough to saturate the alkali is added. The first portions of 
acid dropj)ed into the potash dissolve instantly by combining 
with the potash, and when a fresh addition refuses to dissolve 
after a few moments, it may be concluded that enough has 
been added. The solution is then to be warmed till the 
remaining acid dissolves. Three or four grains of chloride of 


354 


THE CHEMISTRY OF PHOTOGRAPHY. 


gold in solution are then added ; and the whole diluted so as 
to form a hath of eight to twelve ounces.” 

Lea adds that the bath so prepared may either he used at 
once, or will keep well. 

Investigations of Sutton, and of Davanne and Girard 
INTO Toning Processes. 

Thomas Sutton on Toning in 1859 . — Few men held more 
decided ideas upon photographic matters than Thomas Sutton, 
who edited Photographic Notes from 1856 to 1868. 

In his periodical for September 1, 1859 (Yol. lY., p. 217), 
Sutton speaks very clearly and correctly on the subject of 
toning : 

“About the year 1851 M. Le Gray published a method of gold toning 
in which chloride of gold, rendered acid by the addition of muriatic acid, was 
used. The print being first greatly over-printed, was washed and then 
put into this bath, where it was quickly bleached and toned. It was then 
washed and fixed in fresh hypo as usual. No chemical reason was given 
for acidifying the chloride of gold, and it now appears that this was wrong ; 
and that it ought to have been made alkaline instead of acid. Hundreds 
of thousands of prints have been lost through this mistake, for had M. Le 
Gray given the right formula at first, most persons would probably have 
employed it. 

“ I first saw the account of this toning process in Mr. Hennah’s transla- 
tion of Le Gray’s formula, and tried it, but it entirely failed. Then I 
thought it possible that sel d’or might be the right thing, and that Mr. 
Hennah had by mistake translated it into chloride of gold. So I got some 
sel d' or, and it answered perfectly, except that the prints did not require 
over-printing. Then I worked away with the sel d’or process upon plain 
paper — added serum of milk to the salt to give vigor — and washed the 
prints with ammonia to decompose the free nitrate into ammoniacal oxide 
of silver. After some months of experimenting I sent an account of the 
sel d’or process to the P hotographic Journal, and it attracted the attention 
of Mr. Hardwich, and was thought a useful novelty. 

“ But the sel d’or process did not answer upon albumenized paper, and 
that was all the rage ; so albumenized prints were toned in a bath of hypo 
to which chloride of gold was added ; and they have for the most part 
faded. I would observe here that I have known many scl d’or prints fade 
in consequence of the following improper treatment : The acid sel d’or is 
not thoroughly washed out of the print before putting it into hypo ; then 
the acid makes the hypo milky, and the print is sulphurated, and there- 
fore fades. But when the sel d’or process is properly conducted the prints 
do not fade. Not one of my own sel d’or prints have faded. 


THE TONING OF PHOTOGRAPHS, ETC. 


355 


“And now comes the funny part of this history. Someone tried alka^ 
line chloride of gold instead of Le Gray’s acid mixture, and it was found 
to answer capitally, particularly upon albumenized paper. It was not 
until after years of beating about the bush, and after French and English 
chemists had exhausted their resources, and a Printing Committee had 
acknowledged itself beaten by the difficulty of the problem, that the happy 
thought occurred to some one of trying alkaline instead of acid chloride of 
gold. The result is that it answered and solved the problem, and no 
difficulty now remains in getting permanent gold-toned albumenized sun- 
prints.”* » 

Sutton then proceeds to give directions for making and 
using a toning bath of alkaline chloride of gold in a manner 
which is practically identical with the mode employed at the 
present day : 

“Take the common acid chloride of gold, containing hydrochloric acid 
in excess. Dissolve it in water, about half a grain to the ounce. Then 
dissolve a little carbonate of soda in distilled water (the strength is imma- 
terial). Put a strip of litmus paper into the gold solution ; it is quickly 
reddened ; then add the soda solution drop by drop until the blue color 
is restored to the litmus paper. This is the toning bath — and the mode of 
using it is as follows : 

“After removing the print from the pressure-frame wash it thoroughly 
in several changes of water, in order to remove the free nitrate of silver. 
This washing is very important, for if nitrate of silver is introduced into 
the toning bath it throws down chloride of silver and metallic gold, and of 
course destroys the bath. 

“Then put the print into the toning bath. It quickly takes the well- 
known deep purple color due to gold ; but the time depends upon the 
strength and temperature of the bath. With a fresh bath the print is 
toned in about a minute. The lights do not become yellow, but on the 
contrary are bleached ; and if the print is left too long in the bath they 
assume a dull white, which reminds one of putty, at the same time that 
the blacks get too black ; and the print has a sombre disagreeable look. 

“ When the print has been toned, wash it well in several changes of 
water, and then put it into a fresh hypo bath rendered alkaline by the 
addition of a little carbonate of soda or ammonia.” 

The above contribution from the popular pen of Sutton 
doubtless helped materially to introduce alkaline gold toning. 
But the Bev. W. H. Burbank, in his book on ^^Photographic 
Printing Methods,” is hardly correct when he writes (p. 46) : 

*Alas ! Thomas Sutton, we fear you were a little “ too previous ” in making this state- 
ment. We wonder how many of these prints made in 1859 are cow in existence 
unchanged ? Not many. — W. J. H. 


856 


THE CHEMISTEY OF PHOTOGEAPHY. 


‘‘ Hence, the sel d’or bath, as the mixed bath was termed, was 
soon discarded in favor of alkaline solutions of chloride of 
gold, first introduced under the name of Sutton’s Alkaline 
Toning Bath.” To begin with, the “sel d’or” bath is not the 
same as the “ mixed bath ” ; while the introduction of the 
alkaline method is due in the first place to Waterhouse (1855), 
and secondly to Hardwick (1857). 

The Classical Researches of Ravcmne and Girard in 
1863- f — It was reserved for two French chemists and pho- 
tographers — MM. Davanne and A. Girard — to publish, in 
1864,'^ the first complete and scientific research which had 
been made into the theory of toning. 

They begin with a definition : “ The operation to which 
the name of toning is given in photography, has for its object 
the changing the hue of the positive proof, so as to place 
it in the best possible conditions of stability ; and, at the same 
time, to impart to it an agreeable tint.” 

In this definition we note that toning has a dual object. It 
is not merely a coloring operation ; but one in which — by 
replacing one metal by another — (gold ordinarily taking the 
place of silver) a picture possessed of greater elements of per- 
manence is secured. 

One way in which the subject of toning may be considered 
IS under the two heads of 

I. Toning before fixing. 

II. Toning after fixing. 

The latter method is to be avoided because (1) of the extra 
trouble involved by the thorough washing which the print 
would then have to undergo between the two operations ; (2) 
because the gold, in depositing, would cause the formation of 
a certain amount of chloride of silver, which would blacken 
when the print was subsequently exposed to light; a final 
fixing bath would obviate this, but it would take time and 
cause trouble ; (3) the albuminate of silver would undergo an 
injurious change of hue by contact with the hypo before 
toning. 


^Researches sur les epreuves photographiques positives. Paris: Gauthier-Villars. Trans- 
lated in the Photographic News for 1863-4. 


THE TONING OF PHOTOGRAPHS, ETC. 357 

Considering the subject of toning from the point of view of 
the agent employed, we have : 

I. — Sulphur Toning ; as by means of : 

{a) Old hyposulphite. 

(b) Acidulated hyposulphite. 

(c) Hyposulphite charged with salts of silver. 

This method is radically liad, for the presence of sulphur (as 
shown by Davanne and Girard in their memoir read before the 
French Photographic Society, 19th October, 1855) is the 
principal cause of the fading of silver prints. 

II. — Gold Toning. — As toning by means of gold is the 
universally adopted metliod, it must be considered in detail. 

Theory of Toning. — Ordinary toning is effected simply by 
the substitution of one metal for another — gold taking the 
place of silver. It is exactly the same as when a plate of silver 
is dipped into an ordinary gilding solution. Some of the silver 
is dissolved, and gold takes its place. It is never possible, 
however, to effect a complete exchange — the whole of the 
silver is never replaced by gold ; for when the outer layer of 
silver is replaced by gold, this gold protects the silv^er beneath 
it from further action. 

Davanne'’ s Glassification of Gold-Toning Processes. — Four 
classes may be distinguished among the various methods of 
toning by means of gold which have been introduced since 
1850. 

(1) Acid Gold Toning. — By this mode commercial chloride 
of gold is employed, to which a certain quantity of an acid — 
generally hydrochloric acid — is added. This was the method 
practised by Le Gray. The prints are so greatly reduced by 
this bath that to look presentable when finished, they must be 
over-printed until they are nearly black all over. 

(2) Toning by Sel T Or. — In the Photographic Journal for 
IMarch 20, 1855, Thomas Sutton recommends the following 
coloring bath : 

Distilled water .... 30 ounces 

Sel d’or {not chloride of gold) ISi grains 

Pure hydrochloric acid. 1 drachm 

This ^‘sel d’or” is crystallized hyposulphite of gold. It 


358 


THE CHEMISTEY OF PHOTOOEAPHY. 


answers well for prints on plain or matt-surface paper, but not 
for albumenized paper. 

(3) Toning with Neutral Chloride of Gold. — The double 
chlorides of gold and either potassium or sodium are used in 
this method. 

(4) Toning with Alkaline Gold Chloride.- — The double 
chloride of gold and sodium is most frequently employed ; and 
to this are added certain salts having alkaline qualities, such as 
the bicarbonate, acetate, etc., of soda, and chloride of lime. 
This method was first used by Mr. Waterhouse in 1855 ; but 
it was not published until 1857. 

Chemical Analyses of Untoned and Toned Prints^ jper- 
formed hy Davanne and Girard. — A very important feature 
in the work of the two French chemists whose results we are 
now summarizing consisted in the numerous chemical analyses 
which they made of toned and untoned prints. 

These analyses led Davanne and Girard to the following 
conclusions : — 

“1st. — In all toning processes, where no accessory phenomenon inter- 
venes, the replacing of silver by gold takes place in the atomic propor- 
tions required by the nature of the salt of gold employed. 

“2d. — This replacing takes place upon the portions formed of silver 
by simple substitution, and upon the portions formed of silver and argen- 
tino-organic matter by a double decomposition, which forms, in the place 
of the latter, a corresponding auric-organico compound, analogous to the 
combinations which take place in the process of dyeing, between the 
coloring materials and the organic tissues. We also believe that it is to 
this auric-organico combination that the proof owes all its brilliancy. 

“ 3d. — The replacing of the silver by gold takes place equally upon the 
darkest portions as upon the half-tones ; however, it appears to be more 
rapid upon the parts slightly colored, and this result is easily explained 
by the lesser thickness of these parts. 

“ 4th. — The deposit of gold is also much more rapid upon a paper simply 
salted — the picture of which is consequently formed, for the most part, of 
metallic silver — than upon a paper simply albumenized, the picture of 
which is, consequently, almost solely formed of a sort of argentico-organic 
lake, upon which the double decomposition we have spoken of above must 
take place. 

“ 5th. — A comparison of the results furnished by the four classes of 
toning processes we have examined above, shows {a) that the application 
of solutions of gold acidulated with hydrocTiloric acid cannot be performed 
successfully ; {b) that the double hyposulphite of gold and soda (sel d’or), 


THE TONING OF PHOTOGRAPHS, ETC. 


359 


does not give favorable results except in presence of an excess of ammonia 
or by hyposulphite of soda, by which it enters the category of neutral or 
alkaline toning ; and {e) that in fact it is only in the employ of neutral or 
alkaline baths that we should seek the practical conditions of toning.” 

Most of these conclusions have been substantiated by the 
work of other investigators in later years. 



CHAPTER XXXL 


TONING OF PHOTOGRAPHS (CONTINUED). 

Modern Ideas About the Chemistry of Toning. 

Meldola upon Toning . — In the admirable series of lectures 
on the ‘‘ Chemistry of Photography,” delivered at the Fins- 
bury Technical College, London, in 1888, and reprinted as a 
book (published by Macmillan & Co., London, and sold by The 
Scovill Co.) in 1889, we get the ideas upon the chemistry of 
photographic toning held by one of the first of modern 
chemists. 

When chloride of gold is dissolved in hydrochloric acid, a 
compound named chloro-auric acid is formed, thus : 

AuClg + HCl = 

Chloride of Gold and Hydrochloric Acid produce 

HAuCR 

Chloro-Auric Acid. 

This chloro-auric acid is obtained in yellow crystals when 
the solution is evaporated. The chemical composition of 
these crystals is HAUCI 4 , PHgO. These crystals are deliques- 
cent, and when they are dissolved they yield an acid solution, 
which must be neutralized with powdered chalk before it can 
be used for toning. 

The “chloride of gold” usually kept by dealers in photo- 
graphic chemicals is not, however, the above salt, but a double 
salt (NaAuCl 4 , 2 H 2 O), which is obtained by adding a solution 
of common salt to auric chloride and then evaporating the 
liquid to the crystallizing point. This double salt is evidently 
the sodium salt of chloro-auric acid, and may therefore be 
called sodium chloro-aurate ; it is neutral and non-deliquescent. 

Toning consists in so using this sodium chloro-aurate as to 
“deposit on the darkened portions of the unfixed print a 
finely precipitated powder of reduced gold, which changes the 


361 


THE TONING OF PHOTOGEAPHS, ETC. 

reddish color of the mixed reduction products'^ into the tone 
so familiar in finished silver prints.” 

To insure the neutrality of the toning hath we mix with 
the gold chloride various substances, such as chalk, borax, 
or several salts of sodium, as the carbonate, bicarbonate, or 
acetate. 

iN’ow gold is “ reduced to the metallic state with great ease 
from a neutral or alkaline solution.” Let ferrous sulphate, for 
example, be added to a ready-made toning bath, and the gold 
is at once precipitated as a blackish powder on the bottom 
and sides of the vessel in which the experiment is performed : 

2AUCI3 + 6FeS04 -- Aug -f FegClg -f- 

Gold Chloride and Ferrous Sulphate produce Gold aiid Ferric Chloride and 

2 Feg(S 04)3 
Ferric Sulphate. 

iN’ow the “ reduction products” (whatever their exact nature 
may be) present in the untoned, print are ready — and able — 
to play the part of reducing agents. They decompose the 
gold salt, and attract the gold toward themselves. The unal- 
tered silver chloride, etc., possess no such power, and there- 
fore the white parts of the print remain untoned. 

Hyposulphite of soda is a powerful reducing agent, and if 
a very small quantity of it gets into the toning solution it will 
combine directly with the chloride of gold, and prevent its 
precipitation upon the image. Hence the fixing bath ought 
always to be kept at a considerable distance from the toning 
bath ; and after the hands have touched hypo ” they should 
be well washed (and a brush used to dislodge any of this dele- 
terious chemical which may have got under the finger-nails) 
before they are permitted to handle the prints which are in 
the toning bath. 

Thus the toning bath may be considered “ as containing a 
potential deposit of metallic gold ready to be precipitated on 
any reducing surface that may be bathed by it.” The only 
‘‘reducing surface” which we should allow to come in contact 

* These “ reduction products ” are those resulting from the action of the light upon the 
silver chloride and silver albuminate with which the sensitized paper is coated. Accord- 
ing to one theory they are sub-salts ” of silver ; according to another, metallic silver. 


362 


THE CHEMISTRY OF PHOTOGRAPHY. 


with the gold solution is the surface of the print which we are 
desirous of toning. 

Why the Toning Bath should he Prevented from hecommg 
Acid. — Free hydrochloric acid in a toning bath acts as a 
restrainer, preventing the deposition of gold, or allowing it to 
be deposited so slowly that it appears as the red form of the gold 
molecule ; whereas to change the tint of the print (which is red 
to begin with) we desire the molecule of gold to reflect hlue 
light, and to do this the gold must be deposited more rapidly. 

Now free hydrochloric acid is often present in commercial 
chloride of gold ; and it is almost always produced during 
the reduction of the gold chloride by the combination of chlo- 
rine with hydrogen. Thus for example : 

2AUCI3 + SHgO = 6HC1 + AugOg 

Gold Chloride and Water produce Hydrochloric Acid and Gold Trioxide. 

This trioxide of gold is a very easily decomposed substance, 
and its formation is possibly always an intermediate stage be- 
tween the decomposition of the gold chloride and the actual 
deposit of metallic gold on the print. 

To neutralize the ill effects of the free hydrochloric acid is 
the function of the sodium acetate, carbonate, or other salt 
which is added to the toning solution. It effects this by com- 
bining with the hydrochloric acid, the result being the forma- 
tion of a soluble chloride and of some weak acid — such as 
acetic acid or carbonic acid — whose presence is harmless : 

NagCOg + 2HC1 = 2NaCl + 

Sodium Carbonate and Hydrochloric Acid produce Sodium Chloride and 

H2CO3 

Carbonic Acid. 

Why Prints Look Weak After Toning, — Almost every 
instruction book on photography contains a direction to “over- 
print,” to some extent, because the image is weakened by the 
subsequent operations in the toning and in the fixing baths. 
We must now consider why a print should lose any vigor 
because of the chemical action of the toning bath. Abney 
writes 


* “ Instruction in Photography,” p. 262. 


THE TONING OF PHOTOGEAPHS, ETC. 


3,63 


“ Supposing a (silver) print to be thoroughly washed, and immersed in 
a dilute solution of gold trichloride, the following phenomena would pre- 
sent themselves: The picture would gradually bleach, and a blue deposit 
would take the place of the more vigorous red image, and, on immersion 
in the fixing bath, the print would be of the most feeble character.” 

The reason of these changes is this : The chlorine (liberated) 
from the gold chloride would attack the silver siibchloride of 
the print, and — while depositing metallic gold — would in 
reality convert the subchloride forming the image back to the 
state of chloride : 

SAggCl + AuClg = 6AgCl + 

Silver Subchloride and Gold Trichloride produce Silver Chloride and 

Au. 

Gold. 

In this case we see that a single atom of gold has displaced 
six atoms of silver. Of course the single gold atom cannot 
“ make as much show ” as the six atoms of silver did, and the 
print consequently looks very much weaker after toning in 
such a bath than before. 

For this reason we add some substance to the toning bath 
which shall have an equal or greater attraction for the chlorine 
liberated from the gold chloride than the silver subchloride has. 

This brings us to the consideration of another way of classi- 
fying toning baths, viz., into — 

{a) Toning baths in which all the free nitrate of silver is 
removed from the print before toning. 

(^) Baths in which it is an advantage to leave a little free 
silver nitrate in the sensitized paper. 

The acetate bath is a good example of the first of these 
divisions. Sodium acetate has a stronger affinity for the chlo- 
rine contained in the gold trichloride than the silver subchlo- 
ride of the print has. Thus the subchloride is not attacked 
by the chlorine ; and as a result there is little diminution in 
the depth of the print by the subsequent fixing bath. 

In the ordinary ‘‘ lime bath ” we have what is called chlo- 
ride of lime,” but which is really a mixture of calcium chlo- 
ride (CaClg), with calcium hypochlorite (CaClgOg). The 
latter of these two substances acts as a retarder,” preventing 


364 : 


THE CHEMISTEY OF PHOTOOEAPHY. 


the too rapid decomposition of the gold chloride. If prints are 
thoroughly washed and placed in a bath containing nothing 
hut gold chloride and chloride of lime, they will tone very 
slowly and irregularly. 

If a silver print be washed but a little (so that some free 
silver nitrate is left in it), and placed in a solution of plain 
chloride of gold, the toning will be too rajpid to be under 
control. 

But when we get the gold, the lime, and the silver nitrate 
all together, then toning takes place at thq proper 'rate and in 
a regular manner. 

The function of the silver nitrate is to combine with the 
chlorine liberated by the decomposition of the gold chloride : 

HgO + AgNOg -4- CI 2 = AgCl + 

Water and Silver Nitrate and Chlorine produce Silver Chloride ana 

HNO 3 + HCIO 

Nitric Acid and Hypochlorous Acid. 

Thus the chlorine is prevented from attacking the silver 
subchloride of the print. 

Mixed and Miscellaneous Toning Baths. 

Under this head we propose to insert certain formulas for 
toning baths which seem to require separate mention. They 
include those which contain several — or at least more than one 
— additions to the chloride of gold; so that they cannot be 
properly indicated by the name of any one chemical. They 
are also, for the most part, ‘‘well recommended” baths; i.e.^ 
they come to us vouched for by men well known in photog- 
raphy, and as the results of long practice. To find them we 
have turned over many thousands of pages of the literature of 
photography. 

Saeony’s Toning Bath. 


Stock Solution, No. 1. 

Chloride of gold 15 grains 

Distilled water 2 ounces 

Stock Solution, No. 2. 

Carbonate of soda 2 drachms 

Distilled water 2 ounces 


THE TONING OF PHOTOGRAPHS, ETC. 


365 


To tone two sheets of paper, take : 


Stock solution, No. 1 1 drachm 

Stock solution, No. 2 1 drachm 

Warm distilled water (80 deg. Fahr.) 8 ounces 


Add the gold last ; and wait till the mixture is quite color- 
less. This hath (published in 1867) is said to give fine violet- 
- fiblack tones. 

Bovey’s Plain Toning Bath . — The bath next to be described 
appeared in a series of articles on “ Silver Printing,” contributed 
by Mr. W. T. Bovey to the Photographic News for 1868 : 

“Use the orange-colored sample (commercial) of gold; which is a 
double salt, consisting of chloride of gold and sodium. 

“ Keep this gold in concentrated solution, thus : 


Chloride of gold 1 grain 

Distilled water 1 drachm 


“ 1st. Measure out two gallons of water (clear rain or river water if at 
hand ; well-water should be previously boiled). * 

“2nd. Measure into a jug (porcelain) 12 grains of gold ; add about 1 
grain of fine table-salt ; and pour over the whole, I 2 pints of boiling water ; 
allow this to stand awhile until lukewarm ; then add the 2 gallons of 
water previously measured out. Your bath is made, and ready for use. 
Go to work.” 

Bovey adds that this bath improves with age. It may be 
strengthened when needed by pouring -J a pint of boiling 
water over 4 grains of gold, to which J of a grain of fine salt 
has been added ; allow this to cool and then add it to the main 
toning bath. The bulk of the whole bath (2 gallons pints) 
should also be kept up by the addition of pure water when 
necessary. The quantities given are for professional use ; ama- 
teurs should commence with one-half those stated. 

Again, with this bath the “ free nitrate ” must not be all 
washed out of the prints. This being the case it will clearly 
be better not to attempt to save any of the toning bath which 
may be thought to be not quite spent, by returning it to the 
main stock. Only pour out so much as is needed to tone the 
prints in hand, and then throw it away. 

PuranPs Toning Bath . — The Year-Book” for 1876 


* Use distilled water, if possible, for making up every toning solution. — W. J. H. 


366 


THE CHEMISTEY OF PHOTOGEAPHY. 


(p. 110) contains a formula by Mr. C. Durand, for which he 
claims the advantages ‘‘ that a stock bottle of one grain of gold 
to the ounce may be kept for many weeks without depositing 
gold in any appreciable quantity, and it may be added to water, 
or to the weaker solution of it which is in daily use, at a 
moment’s notice, without danger of that form of mealiness 
which is often produced by the* toning bath.” 

Chloride of gold 15 grains 

Lime-water 15 ounces 

Acetate of soda, 3 drachms 

This is the stock solution, and should be made two days be- 
fore it is required for use. It should be kept in a stoppered 
bottle, and for use 1 ounce of it should be added to 6 ounces 
of water. 

After use, the solution should not be returned to the stock 
bottle, but may be thrown away (if exhausted), or placed in a 
second bottle to which more of the stock solution should be 
added when it is again desired to use it. 

CherrilVs Sulphocyanide Toning Bath, — Mr. iN’elson K. 
Cherrill was well known, twenty years ago, as the partner of 
Mr. H. P. Pobinson, and as one of the first professional photog- 
raphers in England. He contributed an account of his favor- 
ite toning bath to the Year-Book” for 1868, p. 62. 

Make up the following solution : 


Chloride of gold 1 grain 

Sulphocyanide of ammonium 20 grains 

Distilled water 2 ounces 


This is the formula as given by Cherrill, but it would, of 
course, be advisable to make up at least eight ounces of the 
solution, multiplying the above quantities by four. 

Ho over-printing is required. Wash the prints thoroughly 
before toning, and use the bath fairly warm — say YO deg. or 
75 deg. Fahr. ‘^The image is first reduced, on immersion, to 
a foxy tone, and then it becomes strengthened, by degrees, to 
a series of colors ; rich warm, and brilliant, ending in black.” 
A good deal of gold is used up by this bath ; about two grains 
per sheet. Tlie bath can be used over and over again, being 


THE TONING OF PHOTOGRAPHS, ETC. 367 

strengthened with gold every time after using. After toning, 
fix in hypo as usual. 

Heisch’s Lime Toning Bath. 

“ Dissolve 1 grain of gold in one drachm of water ; to this add lime- 
water until the blue color is just restored to reddened litmus paper. Now 
dissolve 8 grains of dried, but not fused, chloride of calcium in 5 ounces of 
water ; to this add the solution of gold, stirring all the time ; and, finally, 
add about 3 ounces more of water.” 

In this formula gold ” of course means chloride of gold. 
Heisch was an English chemist and photographer of great ex- 
perience ; he published this formula in 1865. Very little over- 
printing is required, and the prints get blacker as they dry. 
This bath may be used after it has been mixed twenty minutes, 
or on the following day. If it be not exhausted it will keep 
if a drop or two of acid be added (just enough to redden lit- 
mus paper), and then, when again required for use, enough 
lime-water must be added to neutralize this acid. 

Ferguson’s Toning Bath. 

Dissolve 15 grains of chloride of gold (an ordinary ‘‘tube”) 
in 15 ounces of lime-water {iiot chloride of lime), and add 
drachms of acetate of soda to the solution. 

Put the mixture in an earthen pot, stand this in a saucepan 
containing water and put it on the fire until it boils. Keep for 
two days before using. 

The above is the stock solution. Dissolve 4 grains of car- 
bonate of soda in 8 ounces of water, and add to it 1 ounce of the 
stock solution. Keep the bath at a temperature of 70 deg. 
Fahr., by standing the dish containing it in a tin dish half full 
of water. 

Lewis’s Toning Bath. 

Mr. Abel Lewis (a well-known professional) gave the fol- 
lowing toning bath in the “ Year-Book” for 1879, p. 68 ; 


Chloride of gold 2 grains 

Acetate of soda 60 grains 

Saturated solution of chloride of lime 8 drops 

Bicarbonate of soda 1 grain 

Distilled water . . 2 pints 


368 THE CHEMISTKY OF PHOTOGKAPHY. 

He adds : ‘Ht is better to put a number of prints at once 

into a ratber weak solutiou, and let them all tone slowly and 
gradually, than to use the gold in a more concentrated form. 
The prints thus toned have that rich, juicy appearance that 
rapid toning generally destroys.” 

DunmoWs Toying Bath ( 1887 ). 

Print rather deeply in diffused light. Wash the prints in 
plain water till there is no milkiness. Then dip in very weak 
salt-water and wash again. 

Mix the following toning bath a day or two before it is 
required : 


Chloride of gold 1 grain 

Chloride of calcium 4 grains 

Acetate of soda 30 grains 

Distilled water 10 grains 


Immerse the washed prints, and keep them moving. 

Some General Notes on Toning. — Three Common Mis- 
takes IN Toning. 

(1) Taking the print out too soon . — If the print be removed 
from the toning bath at too early a stage, a sufficient deposit 
of gold will not have taken place. The fixing bath dissolves 
out the dark ‘G’eduction products,” and the gold which is 
left is not sufficient to give body ” to the picture. 

(2) Leaving the prints in too long . — When an ordinary 
toning bath is working well, the prints will usually be satis- 
factorily toned in from ten to twenty minutes. If they be 
left in the batli for, say, twice this time, their tone after 
fixing will be a slaty bine, and they will have a feeble and 
“ washed-out ” appearance. The reason is that nearly all the 
silver in the print has been replaced by gold. Now it is to 
the combination of hues afforded by the dark ruddy silver 
underneath, covered over by a layer of bluish gold, that we 
owe the fine purplish-black tones which are esteemed by most 
connoisseurs. But the gold alone., or nearly alone, is unable 
to produce so good an effect. 


THE TOi^ING OF PHOTOGRAPHS, ETC. eS69 

(3) Using a Toning Bath too Strong in Gold . — If too mnch 
gold is used in the toning bath, we get the same bluish feeble 
prints as described in the last paragraph, and from the same 
cause — too complete a replacement of the silver by the gold. 
Our aim in toning must be to coat or gild the silver, and not 
to entirely substitute the gold for it ; for, although the prints 
might in the latter case be more permanent, still a pleasing 
tone must be our first, though not our only aim. As a rule, 
the toning bath should not contain more than one grain of 
gold to ten ounces of water. 

Toning with a Brush. 

Where it is desired to experiment on the properties of a 
toning solution, the method of toning with a brush will be 
found very economical. Wet a piece of clean white paper 
the size of the print, and lay it upon a sheet of glass ; place 
the print to be toned upon this white paper, face upwards. 
Put a little distilled water in a test-tube, and add a drop or 
two of a solution of carbonate of soda, so that the liquid just 
turns red litmus paper blue. Then add one droj) of a solution 
of chloride of gold apply this solution to the print by 
means of a camel-hair brush. If the print is a very large one, 
of course a larger vessel than a test-tube may be used to 
contain the toning solution, which, in such a case, might be 
applied by the broad brush called ‘‘ Blanchard’s brush,” which 
is made by fastening a strip of swan’s-down calico or Canton 
flannel to the end of a strip of glass of the desired width, the 
material being bent over or wrapped round the end of the 
glass ; or a Buckle’s brush ” may be used, which is made by 
drawing a tuft of cotton- wool (by means of a piece of string 
or a silver wire) into the end of a glass tube about half an inch 
wide. It will be found that the small quantity of toning solu- 
tion, made up as described above, will tone well one, or even 
two half -plate prints. This brush-toning ” method was de- 
scribed by Mr. John Hey wood as early as 1859. 

* The strength of this solution is not very material. We always put a fifteen-grain tube 
of gold into a stoppered bottle containing ounces of distilled water, and break the tube 
by shaking the bottle. There is then one grain of gold in each fluid ounce of this, the stock 
solution. 


3Y0 


THE CHEMISTRY OF PHOTOGRAPHY. 


What to do with Old [Intoned Silver Prints. 

When silver prints are kept several days before toning, it 
usually happens that the whites assume a yellowish tint, which 
is unaffected by the subsequent processes of toning and fixing, 
and which mars the beauty of the finished results. 

In such cases an improvement or cure can be effected by 
immersing the prints before toning in a bath containing three 
drachms of ammonia to a pint of water ; wash the prints in a 
similar bath after toning; and a like quantity of ammonia 
should be added to the fixing bath. 

Pules for Toning. 

1. The prints to be toned should be printed slightly darker 
than they are meant to be when finished, and no parts should 
remain quite wdiite. 

2. For the acetate bath, wash the prints thoroughly (includ- 
ing one rinse in very weak salt-water) before toning. For the 
other toning baths, wash in three changes of water only (total 
time of washing not to exceed ten minutes). 

3. Make up the toning bath with distilled water ; use filtered 
rain-water if this cannot be obtained. 

4. The toning bath must not be colder than 60 deg. Fahr. 
(65 to TO deg. Fahr. best — test with thermometer); it must 
also be neutral or slightly alkaline, slowly turning red litmus 
paper blue ; an excess of the alkali is, however, to be avoided. 

5. Immerse each print separately in the toning solution. 
Do not tone more than six prints at a time, and frequently 
move the prints about, placing the bottom print on top, and so 
on. Pock the dish frequently. 

6. Trim the prints before toning. 

7. Wash the prints in three or four changes of water after 
toning. 

8. Pemember that good and black tones can only be ob- 
tained from really good negatives. Be content with brown 
tones from thin or poor negatives. 

9. Always use the same dish for toning ; mark it, and never 
use it for anything else. 


THE TONING OF PHOTOGKAPHS,^ ETC. 371 

10. After your fingers have been in the hypo bath they 
must be well washed and brushed before being again immersed 
in the toning bath. A mere trace of hypo spoils tlie toning. 

11. For ready-sensitized paper, let one of the waters in 
which it is washed before toning contain a little carbonate of 
soda ounce to 1 quart). 

12. In making up a toning bath always add the chloride of 
gold last^ after the other ingredients have been completely dis- 
solved. The bath must never be used until it is quite color- 
less. Keep the acetate bath at least twenty -four hours before 
using. 

13. Judge the tone of a print by weak daylight ; and look 
through the print, holding it up to the light. 

14. Quickly rinse the prints after toning, and then leave 
them to soak in a bath of very weak salt-water (half an ounce 
of salt to a gallon of water); this stops the further toning, 
which would otherwise take place. 

15. Always keep your toning oaths in clean stoppered 
bottles having brown paper pasted round them to exclude the 
light. 

16. Touch the sensitized surface of the paper as little as 
possible with the fingers, especially before the prints are toned. 
The perspiration from the skin prevents the proper action of 
the toning solution and causes reddish marks to appear, which 
are most conspicuous on the dark parts (shadows) of the print. 

17. Freshly-sensitized paper is the easiest to tone and to get 
black tones upon. Keady-sensitized paper does not tone so 
easily after keeping two or three months, and it is often sensi- 
tized upon too weak a bath of silver nitrate. The borax bath 
gives the best results with ready-sensitized paper. 

18. If the prints blister during or after toning, they can 
generally be cured by transferring the prints direct from the 
toning bath into a mixture of methylated spirit four parts and 
water one part. 

19. Paper that is too dry will not give easily toned prints. 
The paper should be kept, if very dry, in a damp cellar, etc., 
for an hour or two before it is put into the printing frame. 

20. Toning baths weak in gold take longer to tone the prints. 


372 


THE CHEMISTfJY OF PHOTOGEA.PHY. 


but produce better tones than baths rich in gold. Never ex- 
ceed the proportion of one grain of gold to eight ounces of 
water ; in most cases the same amount of gold to ten or 
twelve ounces of solution will give even better results. 

The Literatuke of Toning with Gold in Photography. 

From The Photographic Journal (the journal of the Pho- 
tographic Society of Great Britain). 

Pollock., II. — Directions for Obtaining Positive Photo- 
graphs upon Albumenized Paper; Yol. I. (1853), p. 85. 
Maconochie^ A. — Paper Positives, etc. ; Yol. I. (1853), 
p. 87. 

Ilardwicli^ T. F. — On the Chemistry of Photographic 
Printing; Yol. II. (1854), pp. 35, 60, 78. 

Delta. — On Fixing and Coloring Baths; Yol. II. (1854), 
p. 69. 

Sutton., T. — Gold versus Old Hypo; Yol. II. (1855), pp. 
121, 133. 

Fardwichy T. F. — On the Use of Salts of Gold in Photo- 
graphic Printing; Yol. II. (1855), p. 145. 

Sutton., T. — On Positive Printing; Yol. II. (1855), p. 178. 
Davanne^ M. — On the Analysis of Positive Prints ; Yol. II. 
(1855), p. 184. 

Sutton^ T. — On a New Method of Positive Printing; Yol. 
II. (1855), p. 197. 

Davanne and Girard. — On the Kevivificatioh of Faded 
Positives; Yol. II. (1855), p. 199. 

Sidton^ T. — On the Chemistry of Mr. Sutton’s Negative 
Printing Process; Yol. II. (1855), 212. 

Ilardwich^ T. F. — On the Use of Salts of Gold in Photo- 
graphic Printing ; Yol. II. (1855), p. 215. 

Sutton^ T. — On the Hyposulphite of Gold; Yol. II. (1855), 

p. 226. 

IIardwic\ T. F. — On Mr. Sutton’s Process for Toning 
Positives; Yol. II. (1855), p. 244. 

On the Action of Sulphur on Positive Prints; 

Yol. II. (1856), p. 304; and Yol HI. (1856), p. 2. 


THE TONING OF PHOTOGRA.PHS, ETC. 


373 


Caranza^ M. de . — Fixing (.^ Toning) of Positives bv Chlo- 
ride of Platinum; Yol. III. (1856), p. 14. 

Lyte^ F. M. — New Process of Printing; Yol. III. (1856), 
p. 50. 

Hardwick^ T. F. — On Toning Positives; Yol. III. (1856), 

p. 162. 

Shadholt^ George. — A New Toning Process; Yol. III. 
(1857), p. 237. 

Hennah^ T. 11 — On Positive Printing; Yol. IX. (1864), 
p. 36. 

Seeley^ E. — Toning with Gold and Carbonate of Magnesia ; 
Yol. XI. (1886) p. 18. 

T. P. N.— Gold Bath for Dark Yiolets ; Yol. XII. (1867), 

p. 15.. 

Sjoiller, John. — On the Action of Cldoride of Gold upon 
Certain Salts of Silver; Yol. XIY. (1869), p. 91. 

^Yatson., W. H. — Note on Sepia-Toned Silver Prints; Yol. 
XYI. (1874), p. 24. 

Briice^ Geo . — On Printing and Toning Collodio-Chloride 
Paper; Yol. XYI. (1874), p. 47. 

Farmer and Tomjpkins . — Silver-Gold Printing by Develop- 
ment; n. s., Yol. XII. (1888), p. 94. 

From the British Journal of Photography: 

‘‘ Gup .^^ — The Best Method of Printing; Yol. XXXI. (for 
1884), p. 138. 

Burton., IF. K . — Experiments with Silver Prints; Yol. 
XXXI. (for 1884), p. 614. 

. — Notes on Silver Printing; Yol. XXXI. (for 

1884), p. 678. 

Ashman, IF. M. — On Toning; Yol. XXXI. (for 1884), p. 
• 688 . 

{Editorial .) — Toning in Prints Influenced by the Character 
of the Negatives; Yol. XXXII. (for 1885), p. 754. 
Burton, IF. K . — Toning Prints on Albumenized Paper; 

Yol. XXXII. (for 1885), p. 678. 

Stuart, John . — Silver Printing. Yol. XXXII. (for 1885), 
p. 758. 


374 


THE CHEMISTEY OF PHOTOGEAPHY. 


Burton^ W, K. — Gold in Relation to the Permanency of 
Silver Prints; Yol. XXXIII. (for 1886), p. 681. 
{Editorial^ — The Color of Silver Prints; Yol. XXXI Y. 
(for 1887), p. 531. 

Lovejoy^ E. J — Toning of Silver Prints ; Yol. XXXY. (for 

1888) , p. 119. 

{Editorial.) — Amateurs’ Printing Difficulties ; Yol. XXXY. 
(for 1888\ pp. 385, 418, 450. 

Dunmore^ E. — Printing and Other Matters ; Yol. XXX YI. 
(for 1889), p. 8. 

{Editorial^) — Toning by a Brush ; Yol. XXXYI. (for 1889), 
p. 421. 

Barnes., O. B. — Amateurs and Toning; Yol. XXXYI. (for 

1889) , p. 795. 

Bedding^ T. — Toning Bromide Prints; p. 742, for 1890. 
{Editorial^) — Toning Bath; pp. 561, 593, for 1890. 



CHAPTER XXXII. 


THE CHEMISTRY OF PHOTOGRAPHIC “FIXING” 
PROCESSES I. EARLY METHODS. 

To begin with, we must confess that the term “ fixing ” is 
not a very correct one ; and that ‘‘ clearing ” would better 
express what is here meant. When an ordinary dry- 
plate — coated with gelatine containing silver bromide — is 
exposed within the camera, and the picture impressed upon 
it by the lens subsequently developed, we have a great 
number of particles of black metallic silver forming the 
picture, and around and between these are white particles 
of silver bromide which have not been affected by the 
light. The consequence is that the developed (but unfixed) 
picture stands out boldly against its white background and 
border. But if the plate be, in this condition, exposed to day- 
light, the light affects the remaining particles of silver bromide, 
changing them also into metallic silver, or at least into a dark- 
colored compound indistinguishable (to the sight) from it, and 
thus the entire surface of the plate is blackened and all traces 
of the picture are lost. 

Now what is called ‘‘fixing” consists in the removal of the 
unacted on silver bromide from our developed dry-plates; the 
particles of metallic silver forming the picture being then left 
free from any chance of obscuration, and imbedded in the 
colorless gelatine by wdiich they are caused to adhere to the 
glass plate. Silver prints taken from these negatives evidently 
also need fixing for similar reasons. 

For this “fixing,” what is wanted, then, is some substance 
which will dissolve silver bromide or chloride (thereby re- 
moving it from the gelatine film), but which has no action 
upon metallic silver. 

It would seem that ‘^clearing ” would be a better name for 
the process than ‘‘Jixing ^^ ; for in any case the silver remains 


3Y6 


THE. CHEMISTRY OF PHOTOG-RAPHY. 


fixed in the film. What we desire to do is to “clear” the 
molecules of black silver forming the picture from the sur- 
rounding silver bromide. But the term “fixing” has got into 
general use for the operation ; and technical terms — once fairly 
brought into use — are very difficult to displace ; even though 
it can be shown that they are incorrect. 

Wedgwood and Davy Unable to Find a Fixing Agent. 

Thomas Wedgwood, a son of the great English potter, was 
the first man to obtain tolerable copies of more or less trans- 
parent objects by the action of light. This he did by placing 
paper coated with nitrate of silver underneath paintings on 
glass, leaves, the wings of insects, etc. The light passing 
through the substance according to its transparency blackened 
the sensitive surface beneath, leaving it gray where it was 
partly protected and white where it was completely protected 
by the semi-transparent and opaque parts of the substance. 

Sir Humphry Davy (then Professor at the Eoyal Institution 
in London) was the friend of Wedgwood (whose own health 
was exceedingly bad). Davy found that the chloride of silver 
acted rather better than the nitrate, and that it gave better re- 
sults when spread upon leather than upon paper. He em- 
bodied his own and Wedgwood’s experiments in a paper which 
was printed in the Journal of the Royal Institution^ for 1802, 
which paper he winds up with the remark that — “Hothing 
but a method of preventing the unshaded parts of the delinea- 
tions from being colored by exposure to the day is wanting to 
render this process as useful as it is elegant.” Thus, for want 
of a fixing process the work of Wedgwood and Davy was 
rendered useless, and “ photography ” was postponed for nearly 
half a century. 

Hiepce “Fixes” with Petroleum. 

The first man who ever obtained a permanent picture by 
the aid of light was Joseph Hicephore Niepce, a patient and 
persevering worker in France. The key of his discovery lay 
in the fact that bitumen is rendered insoluble by the action of 


THE CHEMISTRY OF FIXING ” PROCESSES. 


377 


light. He coated metal plates with bitumen, which he then 
exposed beneath engravings, and in the camera. Then he re- 
moved the plates and washed them with a mixture of one 
part, bv volume, of the essential oil of lavender, and ten of oil 
of white petroleum.” A thorough washing with water fol- 
lowed. . 

The chemical action of light upon bitumen is to cause it to 
combine with oxygen (from the air) and so to become hard 
and insoluble. But under the opaque lines of the engraving, 
and in the shadows of the camera-picture, the bitumen re- 
mained unoxidized and soluble; and so was washed away by 
the petroleum. In this way a copy of the desired object was 
obtained. But the time required for oxidation of the bitumen 
was terribly long — -six hours to secure jiictures in the camera — 
and while endeavoring to remedy this defect Aiepce died, in 
1833, without ever having published his results. He had, 
however, communicated his experiments, in 1829, to a French 
scene-painter namea Daguerre, with whom he had entered into 
partnership. 

“Sea-Salt” Used as a Fixing Agent by Daguerre. 

There is every reason to believe that the first fixing agent 
employed by Daguerre in his “daguerreotype” process was 
simply a strong solution of sea-salt, or common salt simply, if 
sia-salt could not be obtained. 

HerschePs generous conduct in at once making public his 
knowledge of the excellent qualities of hypo as a fixing agent, 
enabled Daguerre to mention the latter substance in his appli- 
cation for an English patent for his daguerreotype process, 
but he mentions sea-salt first. The following extract is from 
Daguerre’s patent specification dated llth of August, 1839 ; 

''Fifth and last process . — To remove from the plate the coating of 
iodine, and thus to fix the picture, a solution of sea-salt may be used, but 
a weak solution of hyposulphite of soda is preferred. The plate is first 
dipped into distilled water, then moved about in the saline solution until 
the yellow color of the iodine is entirely removed, again plunged into 
water, and finally subjected to the action of a continuous stream of hot 
water falling on an inclined plane carrying the plate, thus cleansing it 


378 


THE CHEMISTRY OF PHOTOGRAPHY. 


perfectly; it is then ready for mounting by being placed in a pasteboard 
case, and covered with glass, thus preserving the silver surface from being 
touched, and from tarnishing." 

Sea-salt is tlie solid matter left by the evaporation of sea- 
water, of which 100 pounds by weight contains no less than 
pounds. More than three-fourths of this solid residue is 
common salt (sodium chloride, JMaCl), but there is also much 
magnesium chloride (MgClg), and some potassium chloride 
(KCl), and magnesium bromide (MgEr^). These substances 
are each and all able to dissolve — to a certain extent, and with 
varying powers — the haloid salts of silver employed in photog- 
raphy. It is doubtful, however, if a photograph, either nega- 
tive or print, was ever perfectly fixed by this means. Doubt- 
less Daguerre knew well the imperfection of his original fixing 
process, and eagerly seized upon that of Herschel. 

Fox Talbot Uses Potassium Iodide and Sodium Chloride 
AS Fixing Agents. 

The first public exhibition of photographs in England was 
on January 25, 1839, when Professor Faraday displayed some 
of the Photogenic Drawings” made by Fox-Talbot, to the 
members of the Poyal Institution of London. Beyond affirm- 
ing that the pictures shown were due solely to the agency of 
light, Faraday said little or nothing. But a few days later — 
on January 31st — Talbot read a paper giving a preliminary 
account of his work up to that date before the Poyal Society 
of London ; and this paper was printed in tlie Pliilosojphical 
Magazine for March, 1839. 

Talbot’s “ Photogenic ” process consisted in impregnating 
paper with silver chloride and nitrate. Although pictures 
could be obtained in the camera by a very long exposure (an 
hour or so) yet it was a printing process mainly. 

Deferring to the work of Wedgwood and Davy, published 
in 1802, Talbot writes*: “The circumstance announced by 
Davy that the paper on which the image v/as depicted was 
liable to become entirely dark, and that nothing hitherto tried 
would prevent it, w^ould perhaps have induced me to consider 


* Philosophical Magazine for March, '\839, vol. xiv., pp. 161-196. 


THE CHEMISTRY OF “ FIXING ” PROCESSES. 379 

the attempts as hopeless, if I had not (fortunately) before I 
read it, already discovered a method of overcoming this diffi- 
culty, and of fixing the image in such a manner that it is no 
more liable to injury or destruction. 

In the course of my experiments directed to that end, I 
have been astonished at the variety of effects which I have 
found produced by a very limited number of different proc- 
esses when combined in various ways; and also at the length 
of time which sometimes elapses before the full effect of these 
manifests itself vdth certainty. For I found that images 
formed in this manner, which have appeared in good preserva- 
tion at the end of twelve months from the time of their forma- 
tion, have nevertheless somewhat altered during the second 
year. This circumstance, added to the fact that the first at- 
tem]3ts which I made became indistinct, in process of time 
(the paper growing wholly dark) induced me to watch the 
progress of the change during some considerable time, as I 
thought that perhaps all these images would ultimately be 
found to fade away. I found, however, to my satisfaction, 
that this was not the case, and having now kept a number of 
these drawings during nearly five years without their suffering 
any deterioration, I think myself authorized to draw conclu- 
sions from my experiments with more certainty.’’ 

From this we see the extreme importance which Talbot 
rightly assigned to the discovery of a fixing agent. It was 
useless to be able to make pictures, unless those pictures were 
capable of being fixed or preserved. Although Talbot’s first 
successful attempts at photography were made as early as 1834, 
yet (from just doubts as to the permanency of his results) he 
did not publish his discovery until 1839 ; and probably would 
not have done so then had not his hand been forced by the 
rumors of Daguerre’s doings in France. 

In the same paper Talbot continues : 

“ At the v^ery commencement oi* my experiments upon this 
subject, when I saw how beautiful were the images which 
were thus produced by the action of light, I regretted the 
more that they were destined to have such a brief existence, 
and I resolved to attempt to find out, if possible, some method 


380 


THE CHEMISTRY OF PHOTOGRAPHY. 


of preventing this, or retarding it as much as possible. The 
following considerations led me to conceive the possibility of 
discovering a preservative process. 

The nitrate of silver, which has become black by the 
action of light, is no longer the same chemical substance that 
it was before. Consequently, if a picture produced by solar 
light is subjected afterwards to any chemical process, the white 
and dark parts of it will be differently acted upon, and there 
is no evidence that after this action has taken place these white 
and dark parts will any longer be subject to a spontaneous 
change ; or, if they are so, still it does not follow that that 
change will now tend to assimilate them to each other. In 
case of their remaining dissimilar, the picture will remain 
visible, and therefore our object will be accomplished. 

If it should be asserted that exposure to sunlight would 
necessarily reduce the whole to one uniform tint and destroy 
the picture, the onus prohandi evidently lies on those who 
make the assertion. If we designate by the letter A the ex- 
posure to the solar light, and by B some indeterminate chem- 
ical process, my argument was this : Since it cannot be shown 
a priori that the final result of the series of processes ABA 
will be the same with that denoted by A B, it will therefore 
be worth while to put the matter to the test of experiment, 
viz., by varying the process B until the right one be discov- 
ered, or until so many trials have been made as to preclude all 
reasonable hope of its existence. 

My first trials were unsuccessful, as indeed I expected ; 
but after some time I discovered a method which answers per- 
fectly, and shortly afterwards another. On one of these more 
especially I have made numerous experiments; the other I 
have comparatively little used, because it appears to require 
more nicety in the management. It is, however, equal, if not 
superior, to the first in brilliancy of effect. 

This chemical change, which I call the preserving process, 
is far more effectual than could have been anticipated. The 
paper, which had previously been so sensitive to light, be- 
comes completely insensible to it, insomuch that I am able to 
show the Society specimens which liave been exposed for an 


THE CHEMISTKY OF FIXING ’’ PROCESSES. 381 

hour to the full summer sun, and from which exposure the 
image has suffered nothing, but retains its perfect whiteness.” 
After reading these paragraphs one can only exclaim ; 
‘‘ Bravo, Talbot ! spoken like a scholar and a philosopher.” 
Davy’s failure did not daunt him, and by the aid of math- 
ematics (Talbot graduated with high honors at Cambridge 
University in 1821) he is able to demonstrate that a fixing 
agent is not an impossibility, and he goes for it. 

To continue our quotations from this epoch-making paper of 
1839: 

On the Art of Fixing a Shadow — The phenomenon 
which I have now briefiy mentioned appears to me to par- 
take of the character of the marvellous, almost as much as any 
fact which physical investigation has yet brought to our 
knowledge. The most transitory of things, a shadow, the pro- 
verbial emblem of all that is fieeting and momentary, may be 
fettered by the spells of our ^ natural magic,’ and may be 
fixed for ever in the position which it seemed only destined 
for a single instant to occupy. 

“ This remarkable phenomenon, of whatever value it may 
turn out in its application to the arts, will at least be accepted 
as a new proof of the value of inductive methods of modern 
science, which by noticing the occurrence of unusual circum= 
stances (which accident, perhaps, first manifests in some small 
degree), and by following them up with experiments, and vary- 
ing the conditions of these until the true law of nature which 
they express is apprehended, conducts us at length to conse- 
quences altogether unexpected, remote from usual experience, 
and contrary to almost universal belief. Such is the fact, that 
we may receive on paper the fieeting shadow, arrest it there, 
and in the space of a single minute fix it there so firmly as to 
be no more capable of change, even if thrown back into the 
sunbeam from which it derives its origin.” 

In this, his first paper, Talbot describes his results only, and 
gives no details as to his methods for obtaining them. But 
two months later* he supplied this want in the form of an 
“open letter” addressed to S. H. Christie, Esq., the Secretary 


Philosophical Magazine for March, 1839, pp. 209-211. 


382 


THE CHEMISTRY OF PHOTOGRAPHY. 


of the Royal Society. He says that the subject naturally 
divides itself in two heads, viz., the preparation of the paper, 
and the means of fixing the design.” 

It is the latter only of these topics which concerns us at 
present. 

^^Method of Fixing the Image .- — After having tried am- 
monia, and several reagents, with very imperfect success, the 
first thing which gave me a successful result was the iodide of 
potassium much diluted with water. If a photogenic picture* 
is washed over with this liquid, an iodide of silver is formed 
which is absolutely unalterable by sunshine. This process re- 
quires precaution, for if the solution is too strong, it attacks 
the dark parts of the picture. It is requisite, therefore, to find 
by trial the proper proportions. The fixation of pictures in 
this way, with proper management, is very beautiful and 
lasting. The specimen of lace which I exhibited to the Soci- 
ety, and which w^as made five years ago, was preserved in this 
manner. 

‘^But my usual method of fixing is different from thiSj 
and somewhat simpler, or at least requiring less nicety. It 
consists in immersing the picture in a strong solution of 
common salt, and then wiping off the superfluous moisture and 
drying it. It is sufficiently singular that the same substance 
which is so useful in giving sensibility to the paperf should 
also be capable under other circumstances, of destroying it ; 
but such is, nevertheless, the fact. 

“ How, if the picture which has been thus washed and dried, 
is placed in the sun, the white parts color themselves of a pale 
lilac tint, after which they become insensible. Humerous ex- 
periments have shown to me that the depth of this lilac tint 
varies according to the quantity of salt used, relatively to the 
quantity of silver. But by properly adjusting these, the 
images may, if desired, be retained of an absolute whiteness. 
I find I have omitted to mention that those preserved by iodine 
are always of a very pale primrose yellow, which has the ex^ 

* A “photographic print ” as we should style it. — W. J. H. 

t Talbot’s “photogenic paper” was prepared by dipping it first into a weak solution 
of common salt, and then into a solution of nitrate of silver so as to have a slight excess 
of the latter substance. 


383 


THE CHEMISTRY OF FIXING ” PROCESSES. 

traofdinarj and very remarkable property of turning to a full 
gaudy yellow whenever it is exposed to the heat of a fire, and 
recovering its former color again when it is cold.” 

We have quoted Talbot’s descriptions somewhat fully, as the 
original is likely to be inaccessible to most of our readers, and 
his words have not, we believe, been previously reprinted in 
any photographic journal. It is always better to get face to 
face with the man who did it,” and to read his own words 
rather than a paraphrase of them. 

Of course, not one of Talbot’s early pictures could have 
been properly “fixed” in the sense in which we understand 
the term. Some of the silver was no doubt washed out of the 
paper, and what was left was prevented from blackening on 
exposure to light by the presence of an excess of a haloid salt, 
either potassium iodide (KI), or common salt (sodium chloride, 
KaCl). 

Talbot’s first or “photogenic” method must be carefully 
distinguished from his “calotype” process which he patented 
in 1841. The former consisted of silver chloride upon paper ; 
the later of silver iodide. The image on the former was 
2 ?rinted out ; on the latter it was developed. 

Talbot also mentions that he sometimes used potassium bro 
mide (KBr) as a fixing agent. 

Fixing with Ammonia by Fyfe and Others. 

We have seen that Talbot attempted to use ammonia as a 
fixing agent for his “photogenic drawings,” but with very 
indifferent results. 

Immediately after the publication of Talbot’s paper, Dr. 
Andrew Fyfe, Yice-President of the Society of Arts, Edin- 
burgh, appears to have experimented on the subject, and he 
read some account of his results to the Society of which he 
was an officer, on March 27, and April 10 and 17, 1839. This 
paper — which is an important one — was published in the 
Philosophical Magazine for the same year. The following 
quotations will prove that Dr. Fyfe was an original and 
earnest worker : 


384 


THE CHEMISTEY OF PHOTOGEAPHY. 


Peeseevation of the Impeessions. 

“ It is evident that, as the impression is produced by the 
Agency of light on the compound of silver, when the paper is 
again exposed, the light will begin to act, and ultimately 
darken the whole, thus effacing the impression ; hence the 
necessity of a preservative process. Two methods have been 
recommended by Mr. Talbot as applicable to the chloride, one 
by the iodide of potassium, the other by sea-salt. When solu- 
tion of iodide of potassium is added to that of lunar caustic,* 
a yellow iodide of silver is thrown down. The same is the 
case when the iodide is put on paper, previously covered with 
the chloride, and, provided the solution is strong, it acts also 
on the chloride when darkened, thus converting it to yellow 
iodide, which is not in the least affected by light; hence, by 
putting the paper with the impression through a solution of 
the iodide, provided it is weak, the white chloride only is acted 
on, and being converted to iodide, it is no longer liable to 
change. As, however, the iodide will act on the dark chlo- 
ride, it is of the utmost consequence to attend to the strength 
of the solution, which should be such that it will not attack 
the faint parts of the impression. After the paper is passed 
through it, it should be kept for some time in water to wash off 
the superfluous iodide of potassium, which, if left on, would 
gradually destroy the whole of the impression ; indeed, even 
with this precaution, I find it extremely difl&cult to preserve 
them. 

“ The second method recommended by Mr. Talbot is merely 
immersing the paper in solution of sea-salt. This process does 
not, however, seem to answer well ; I have repeatedly failed 
in preserving the specimens in this way, and even when they 
are preserved they are completely altered in their appearance 
and deprived of their original brilliancy. 

“ I have already stated that I prefer the phosphate of silver 
for taking the impressions, not only because it is equally sensi- 
tive as the chloride, but gives a greater variety of shades. In 
addition to these it has another advantage : the impressions 


* Nitrate of silver.— W. J. H. 


THE CHEMISTEY OF “ FIXING ” PKOCESSES. 385 

are easily preserved. After various fruitless attempts I at last 
found that the darkened phosphate is not soluble in ammonia, 
though, as is well known, the yellow phosphate is easily dis- 
solved. I had, therefore, recourse to this for their preserva 
tion, and though I did not completely succeed at first, yet 
at last I did so by attending to the precaution of washing oft* 
the ammoniacal solution ; because, wdien left on, the impression 
gradually becomes darker and darker, and is ultimately de- 
stroyed, owing to the action of the light on it. The method 
I now follow is to put the paper into a diluted solution of 
water of ammonia (one of the spirit of hartshorn to about six 
of water) and leave It there till the yellow parts become white, 
showing that the phosphate is dissolved, after which it is 
washed with water to carry oft the whole of the ammoniacal 
solution. It should then, when nearly dry, be subjected to 
pressure till dried, by which it is prevented from wrinkling, 
and the impression retains its original sharpness, which, unless 
this is done, it is apt to lose, by the fibre of the paper being 
raised by the repeated moistening. 

“Though the phosphate specimens may be preserved in this 
way, yet they do not retain exactly their original appearance. 
Those parts, whitened by the ammonia, gradually acquire a 
faint reddish tinge — but, though altering the appearance, it 
does not aftect the brilliancy; indeed, in some cases, it rather 
improves it, by giving it a pleasing tint, which contrasts well 
wdth the darker parts, and gives the appearance of coloring. 
I have also found that carbonate of ammonia answers equally 
well, and, being much cheaper, it will of course be preferred. 
I generally employ a solution prepared by dissolving one part 
of the salt in about four of water, in which the paper is kept 
for a minute or so, and then afterwards washed, and subjected 
to pressure, as already noticed. Impressions thus preserved 
acquire the same reddish tinge as those acted on by ammonia. 

“ I have before stated that the paper may be prepared by 
washing it over with a solution procured by adding nitrate of 
silver to carbonate of ammonia. The impressions taken with 
that paper are easily preserved, by merely washing them with 
water, to carry off the part not acted on by the light, which is 


386 


THE CHEMISTRY OF PHOTOGRAPHY. 


another advantage, in addition to those stated, for using the 
carbonate solution. Like the phosphate specimens, they also 
acquire a reddish tint. 

“ Other preservative methods have been recommended, as, 
by covering the impressions with yellow color, to prevent, as 
much as possible, the transmission of the chemical ray of the 
light ; but those above stated, particularly where the phos- 
phate or carbonate is used, are so simple and efficacious that it 
is unnecessary to allude to them.” 

Dr. Fyfe’s name and work are well worth preserving. He 
must have been among the first — was probably the first — to 
]>ractise photography in Scotland. His fixing process with 
ammonia was better than any fixing method discovered by 
Talbot. Silver chloride is freely soluble in ammonia, but not 
silver iodide or silver bromide. Thus ammonia can only be 
used for fixing prints. 

Another early experimenter — J. C. Constable — arrived at 
the same result as Fyfe. The following letter from Constable 
appeared in the June number of the Philosophical Maqazlne 
for 1839 : 

“Mr. Fox Talbot, in his paper on photogenic drawing, 
states, that he did not succeed in preserving the drawings by 
means of ammonia ; some experiments which I have made 
lead to a different result. I find that the drawings, after 
being soaked for some minutes in a moderately strong solu- 
tion of ammonia, and then washed in clean water, withstand 
the action of the light perfectly, and indeed are improved by 
it ; for the first action of the ammonia is to make the dark 
parts of a reddish hue, which, on exposure to the light, become 
again of a dark color, the light parts being unaffected. This 
mode of preservation has, I conceive, advantages over those 
already used. Common salt never preserves completely so as 
to enable the drawing to withstand the action of the sun. 

“ Iodide of potassium seems to require great delicacy of 
management, as when at all too strong it eats out the fainter 
tints, and is moreover subject to this inconvenience — that 
sometimes the drawings so preserved, even when kept in the 
dark, become entirely bleached and lose all traces of the dark 


387 


THE CHEMISTRY OF FIXING ” PROCESSES. 

lines. This at least has happened to some drawings so pre- 
pared by a friend of mine. There is no doubt that the hypo- 
sulphite of soda is an excellent preservative, but it is a salt 
nut easily prepared, and not likely to be in the hands of those 
who may wish to make experiments on the subject.” 

Hypo at that time cost a guinea a pound, and was wdth diffi- 
culty procurable even at that price. For several years after- 
wards its price did not fall below six shillings per pound. It 
is now sold, in bulk, at about the same price per hundred- 
weight ! The early experimenters deserve our hearty sympa- 
thy. They had difficulties to contend with that we know 
nothing of. Photography w^as then an infant science, and the 
hrst workers had to painfully groj)e their way in the dark, 
and to first invent and then manufacture the apparatus they 
employed ; pure chemicals were almost unattainable, and their 
price was prohibitive. Surely every worker of to-day ought 
to feel a deep interest in the way in which the foundation 
stones of photography were laid, half a century ago. 

The powerful odor of ammonia, together with the announce- 
ment by Herschel of the splendid Affixing” powers of hypo- 
sulphite of soda, prevented the volatile alkali ”* from com- 
ing Into use for fixing photographs. But its claims have 
been recently revived by Mr. K. H. Bow.f He states that the 
advantages of ammonia are: (1) Shortness of the time com 
sumed between the toning bath and the finishing of the print, 
which may be less than ten minutes ; (2) freedom of the print 
from any sulphur compounds, and consequent promise of per- 
manency; (3) great saving of water required for the wash- 
ings ; (4) preservation of fainter shadings, which become 
bleached to a great extent under treatment with the hyposul- 
phite ; (5) the cost may be less if the ammonia and dissolved 
chloride of silver and other silver compounds be recovered by 
partial distillation and treatment with hydrochloric acid ; (6) 
the paler shadings in the picture retain a w^armer tint than in 
the hypo-fixed prints. This, in many cases, will be thought by 
some an advantage, as in portraits and sunny landscapes. 


* A chemical term for ammonia.— W. J. H. 
t British Journal of Photography for April 15, 1887. 


388 


THE CHEMISTRY OF PHOTOaEAPHY. 


For fixing, almost any strength of ammonia may be used, 
from a 10 per cent, solution to one which is forty times 
weaker. The only difference is in the time required. The 
best plan is to use two baths — the first, say, a 2 per cent, solu- 
tion, and the second bath half this strength only — and allow 
the prints to remain for five minutes in the first bath and for 
ten minutes in the second. A final washing in water for ten 
minutes should be given. 

This ammonia fixing bath is well worth a trial ; it is, indeed, 
to be recommended where the time is very limited. 



CHAPTER XXXIII. 


THE CHEMISTRY OF “FIXING” PROCESSES (CON- 
TINUED).— II. “HYPO,” “CYANIDE,” AND 
WATER AS FIXING AGENTS. 

Hypo ’’ and Hersciiel. 

Tlie claims of Sir John Herschel to a high position among 
the “ fathers of photography ” have scarcely received full 
recognition. It is greatly to he regretted that the Herschel 
family has never thought tit to prepare, or to aid others in 
preparing, a full biography of either Sir John Herschel or his 
father. Sir William Ilerschel. But the following extracts 
from one of Sir John’s private note-books will show what he 
had done before the publication of anything about the photo- 
graphic processes either of Daguerre or of Talbot. At that 
time rumors, but rumors only, were spread about that pictures 
had been obtained by some secret method by which objects 
were caused to draw their own likeness”; but Talbot’s first 
description of his own method was only made to the Royal 
Society on January 31, 1839, while Daguerre’s account of his 
process did not appear until August of the same year. 

IlerscheVs Note-Book. — Experiment 1012, made January 
29, 1839. Experiments tried within the last few days since 
hearing of Daguerre’s secret^ and also that Fox Talbot has got 
something of the same kind.” (Here follow some trials of the 
relative sensitiveness to light of the nitrate, carbonate, acetate, 
and chloride of silver.) 

“ Experiment 1013. — Daguerre’s process : Attempt to imi- 
tate. Requisites — 1st, very susceptible paper; 2d, very per- 
fect camera ; 3d, means of arresting further action. 

‘‘ Tried hyposid2)hite of soda to arrest the action of light 
hy washing away all the chloride of silver or other silvering 
salt; succeeds perfectly. 


390 


THE CHEMISTRY OF PHOTOGRAPHY. 


Papers half acted on, half guarded from the light by 
covering with pasteboard, were withdrawn from sunshine, 
sponged over with hyposulphite, then washed in pure water, 
dried, and again exposed. The darkened half remained dark, 
the white half white, after any exposure, as if they had been 
painted with sepia. 

“ January 30, 1839. — Formed image of telescope with the 
aplanatic lens, and placed in focus paper covered with carbon- 
ate of silver. An image was formed in white on a sepia- 
colored ground, which bore washing with hyposulphite of 
soda, and was then no longer alterable by light. Thus Da- 
guerre’s problem is so far solved. 

Experiment 1014, January 30th. — Tried transfer of print 
and copper-plate engraved letters.” 

It may be objected that these were private notes ; but on 
March 14, 1839, Sir John Herschel read a paper before the 
Royal Society of London, from the published abstract of which 
the following quotation is taken : 

Confining his attention in the present notice to the em- 
ployment of chloride of silver, the author inquires into the 
method by which the blackened traces can be preserved; 
which may be effected, he observes, by the application of any 
liquid capable of dissolving and washing oE the unchanged 
chloride, but leaving the reduced oxide of silver untouched. 
These conditions are best fulfilled by the liquid hyposulphites. 

“ Twenty-three specimens of photographs made by Sir J. 
Herschel accompany his paper, one a sketch of his telescope 
at Slough.” 

Concerning this abstract of his paper. Sir John Herschel 
wrote, a quarter of a century later (viz., in 1864) : “ This is 
the image above mentioned as having been taken on January 
30, 1839, and was, I believe, the first picture qyqy fixed for an 
optical image ever taken in this country — at least I have heard 
of none earlier. At the time of making these experiments, as 
already mentioned, I had no knowledge of M. Daguerre’s 
process further than the mention of the existence of a process 
(a secret one) in a note from Admiral (then Captain) Beaufort, 
some time about January 23, 1839. Of course I used paper y 


391 


THE CHEMISTEY OF “ FIXING ” PROCESSES. 

not silver, and it was not a suggestion^ but a regular uniform 
joractice to use the hyposulphite; I never used anything 
else.” 

It was not likely that Herschel would use ‘‘ anything else,” 
for he was the first to call the attention of chemists to the hy- 
posulphites and to their peculiar ]30wer of dissolving the salts 
of silver. It is true that hyposulphite of soda was prepared 
by Chaussier as early as 1799, but Herschel was the first to 
properly study this salt and the acid from which it was de- 
rived. His paper “ On the Hyposulphurous Acid and Its 
Compounds,” which appeared in Brewster & Jamieson’s Edin- 
hurgli Philosojohical Journal for 1819, contains the following 
passages : 

“ One of the most singular characters of the hyposulphites 
is the property their solutions possess of dissolving chloride 
of silver and retaining it in considerable quantities in perma- 
nent solution. 

IIyj)osulj)hite of Potash. — It dissolves chloride of silver, 
even when very dilute, with great readiness. 

‘‘ HyposulgJite of Soda. — Chloride of silver newly precipi- 
tated dissolves in this salt when in a somewhat concentrated 
solution in large quantity, and almost as readily as sugar in the 
water. 

“ Hyposulphite of Strontia. — Like the rest of the hypo- 
sulphites it readily dissolves chloride of silver, and alcohol 
precipitates it as a sweet syrup. 

Ilyposidphite of Silver. — Chloride of silver newly precipi- 
tated is soluble in all liquid hyposulphites, and, as before ob- 
served, in that of soda with great ease and in large quantities. 
This solution is not accomplished without mutual decomposi- 
tion, as its intense sweetness proves — a sweetness surpassing 
that of honey, and diffusing itself over the whole mouth and 
fauces, without any disagreeable or metallic flavor.” 

Second and third papers from Herschel’s pen on the same 
subject appeared in the same periodical for 1819. In the sec- 
ond paper he states that the chemical affinity of hyposulphu- 
rous acid for silver is so great that oxide of silver readily de- 
composes hyposulphite of soda, and even caustic soda — the 


392 


THE CHEMISTRY OF PHOTOGRAPHY. 


only instance, I believe, yet known of the direct displacement 
of a fixed alkali in the wet way by a metallic oxide. 

“ Hyposulphite of Ammonia and Silver . — Its sweetness is * 
unmixed with any other flavor, and is so intense as to cause a 
pain in the throat. One grain communicates a perceptible 
sweetness to 30,000 grains of water.” 

Writing in 1864, Herschel remarks as to these discoveries 
made by him nearly half a century previously : 

The very remarkable facts then described, I have reason to 
believe, attracted a great deal of attention at the time, and 
thenceforward the ready solubility of silver salts, usually re- 
garded as insoluble, by the hyposulphites, was familiar to every 
chemist. It would not, therefore, be surprising if Daguerre 
tried it to fix his plates {i.e., to wash off the iodide coating) ; 
but I have been informed, though I cannot cite a printed 
authority for it, that at first he fixed with a strong solution of 
common salt. For my own part, the use of the hyposulphites 
w^as to myself the readiest and most obvious means of pro- 
cedure, and presented itself at once. My earliest experiments 
were made in January, 1839.” 

Hypo Adopted by Daguerre and by Talbot as a Fixing 

Agent. 

The superiority of Herschel’s fixing method was patent to 
all ; and after his publication of it in the spring of 1839 it was 
adopted both by Daguerre and by Fox Talbot. 

In Daguerre’s English patent (August 14, 1839), he says : 
Fifth and last process . — To remove from the plate the coat- 
ing of iodine, and thus to fix the picture, a solution of ‘sea- 
salt’ may be used ; but a weak solution of hyposuljihite of 
soda is preferred. The plate is first dipped into distilled 
water, then moved about in the saline solution until the yellow 
color of the iodine is entirely removed, again plunged into 
water, and finally subjected to the action of a continuous 
stream of hot w^ater falling on an inclined plane carrying the 
plate, thus cleansing it perfectly.” 

The first book ever written on photography is Daguerre’s 
“ History and Practice of Photogenic Drawing,” wdiich 


THE CHEMISTRY OF “ FIXIYO ” PROCESSES. 


393 


was translated from the French by J. S. Memes, and pub- 
lished in England towards the end of 1839 (the preface is 
dated September 13th). On jiage 65 we find: “Fifth Opera- 
tion; Fixing the Imjjression. The object of this final process 
is to remove from the tablet the coating of iodine, which con- 
tinuing to decompose by light would otherwise speedily 
destroy the design when too long exposed. For this operation 
the requisites are : , 

“ A saturated solution of common salt ; or a weak solution 
of hyposulphite of pure soda. 

“ The apparatus represented, Plate YI., Fig. 4, first and 
second views. Two square troughs, sheet copper, Plate YI., 
Fig. 2, both views. 

“A vessel for distilled water, Plate YI., Fig. 5. 

“In order to remove the coating of iodine, common salt 
is put into a bottle with a wide mouth, which is filled one- 
fourth with salt and three-fourths with pure water. To dis- 
solve the salt shake the bottle, and when the whole forms a 
saturated solution, filter through paper. This solution is 
made in large quantities beforehand, and kept in corked 
bottles. 

“ Into one of the square troughs pour the solution, filling it 
to the height of an inch ; into the other pour in like manner 
your water. This solution of salt may be replaced by one of 
hyposulphate * of soda, which is even preferable, because it 
removes the iodine entirely, which the saline solution does 
not always accomplish, especially when the sketches have 
been laid aside for some time between the fourth and fifth 
operations. It does not require to be warmed, and a less 
quantity is required. 

“First, the plate placed in common water, poured into a 
trough, plunging and withdrawing it immediately, — the sur- 
face merely requiring to be moistened — then plunge it into 
the saline solution, which latter would act upon the drawing 
if not previously hardened by the washing in pure water. 
To assist the effect of the saline solutions, the plate is moved 


* I have preserved t±iis venerable misprint of ate for ite^ because it v\ms probably the 
first ever made. — W. J. H. 


394 


THE CHEMISTRY OF PHOTOGRAPHY. 


about in them by means of a little hoop of copper wire, Plate 
VI., Fig. 3. When the yellow color has quite disappeared, 
the plate is lifted up with both hands, care being taken not 
to touch the drawing, and plunged again into the first trough 
of pure water. 

‘‘Next, the apparatus, Plate YI., Fig. 4, two views, and the 
bottle. Fig. 5, having been previously prepared, made very 
clean, and the bottle filled with distilled water, the plate is 
withdrawn from the trough, and being instantly placed upon 
the inclined plane, Plate YI., Fig. 4, distilled water, hot but 
not boiling, is made to fiow in a stream over its whole surface, 
carrying away every remaining portion of the saline wash.* 

“Not less than a quart of distilled water is required when 
the design is of the dimensions indicated in the engraving, 
8|- by 6-|- inches. The drops of water remaining on the plate 
must be removed by forcibly blowing upon it, for otherwise 
in drying they would leave stains on the drawing. Hence 
also will aj^pear the necessity of using very pure water, for if 
in this last washing the liquid contains any admixture of 
foreign substances, they will be deposited on the plate, leaving 
behind numerous and permanent stains. To be assured of the 
purity of the water, let a drop fall upon a piece of polished 
metal ; evaporate by heat, and if no stain be left the water is 
pure. Distilled water is always sufficiently pure without this 
trial. 

“ After this washing the drawing is finished.” 

Fox Talbot does not seem to have adopted “ Herschel’s 
Hypo ” as a fixing agent with the same speed as Daguerre. 
In Talbot’s patent for the calotype process (February 8, 1841) 
we find : 

“ The Fixing Process . — The picture is dipped into water, 
partly dried, washed with a solution of bromide of potassium 
or some other soluble bromide, washed with water, and finally 
dried.*” 

But two years later (June 1, 1843) we actually find Talbot 
jpatenting the discovery of Herschel. The patent was for nine 

* If hyposulphite has been used, the distilled water need not be so hot as when common 
salt has been employed. 


395 


'THE CHEMISTET OF FIXING PEOCESSES. 

improvements in the calotjpe process : 1st. To give increased 

whiteness to calotype and other photographic pictures, and at 
the same time make them more permanent, they are plunged 
into a hot solution of hyposulphite of soda (or any other 
soluble hyposulphite), then removed, washed and dried.” In 
tlie ninth improvement ” we are told that: The negative 

copy upon ‘ copying paper ’ is fixed by being washed wfith 
warm water, placed in a solution of hyposulphite of soda, and 
all removed that is soluble in water by plunging it into two or 
three baths of warm water consecutively.” 

Of course, such a patent could not be valid ; but it has, 
unfortunately, been too much the practice for the English 
Patent Office to grant letters-patent without making any 
proper inquiry into the originality of the invention claimed. 
Talbot never attempted to enforce this claim for the use of 
hypo as a fixing agent. 

Talbot does not seem to have been well “ advised ” in the 
procuration of his patents, for on December 12, 1849, we find 
him including in a patent a claim for : A method of obtain- 

ing more complete fixation of photographic pictures upon 
paper. In addition to the usual fixing process, the picture is 
dipped into a boiling solution of caustic potash.” This par- 
ticular point, however, we find him disclaiming ” on May 1, 
1855. 

How TO Fix Photogeaphs. 

The “ hypo ” bought should be in clear crystals, and should 
not be too cheap. In England “ hypo ” can be bought at from 
one to six cents per pound ; and the better quality is worth the 
money. Thoroughly pound the crystals in a mortar, and 
make up the solution in the proportion of four ounces of solid 
hypo to twenty ounces of ordinary water. Add a teaspoonful 
or so of ammonia to every quart of the solution ; for acid hypo 
is a thing to be avoided, and the alkali also helps to prevent 
blistering. A solution of this strength (1 to 5) is just right 
for fixing negatives. But for paper prints it should be 
reduced by adding half as much more water, so as to bring 
down the strength to 1 to 74. 


396 


THE CHEMISTRY OF PHOTOGRAPHY. 


When a negative or a print is placed in an ample supply of 
such a solution, the result is the formation of silver sodium 
hyposulphite : 

AgBr + Na^S^Og = AgNaS^Og + 

Silver and Sodium produce Double hyposulphite and 
bromide hyposulphite of silver and sodium 

NaBr 

Sodium bromide. 

This double hyposulphite is very soluble in water, and is 
easily washed out of the him. 

But if an insufficient amount of hypo be employed another 
compound (Ag3Na4, 3 [S2O3]) is formed, which insoluble 
and cannot be got rid of. Let a sensitized collodion (wet) 
plate be half -immersed in a strong solution of hypo for hve 
minutes ; it will become quite clear, all the silver being dis- 
solved out. Now immerse the other half of the plate in a 
very weak (say, one per cent.) solution of hypo, and it will 
be seen to be covered with a blackish deposit of the insoluble 
double salt — Ag3Na4, 3 (SgOg). 

It may be reckoned that one ounce of solid hypo will hx 
three sheets of sensitized paper, or half a dozen quarter-plate 
negatives. Each negative should be left in the solution until 
it loolts quite clear, for which about ten minutes is usually 
necessary. It should then be removed to a second bath of 
hypo, and left there for the same length of time. As soon as 
the first bath begins to fix slowly, it should be thrown away. 
The same plan should be adopted for prints. It is probable 
that much of the fading so commonly seen in both negatives 
and prints is due to insuflcient fixation. It is easy enough to 
note the disappearance of the white silver salt from a negative, 
b}' looking at the bacli of the plate or film. But with paper 
prints the only guide is a greater clearness and transparency 
of the print, as seen when looking through it. In each case 
the best plan is the use of frequently renewed solutions ; and 
the use of two baths, with an adequate time (not less than ten 
minutes) in each. 

Vessels of tin or zinc should never be used as fixing baths; 
the hypo corrodes them; and the prints then stain when they 


THE CHEMISTRY OF FIXING ’’ PROCESSES. 397 

touch the corroded surface. Baths made of lead, with vertical 
grooves are convenient. 

Should the ordinary fixing bath become acid the following 
chemical reaction will take place. 

NasSaOg + 2HC1 = S + SOg 

Sodium and Hydrochloric produce Sulphur and Sulphur 

hyposulphite acid dioxide 

+ 2NaCl + HsO 

atid Sodium chloride and Water. 

Other acids act in a similar way to the one — hydrochloric 
acid — here selected as an example. The suljihur deposited 
will combine with the finely divided black silver which forms 
the picture, to produce sulphide of silver, a compound which, 
when in extremely thin layers, has a yellowish hue. This is 
the reason why the simple hypo bath should always be kept 
alkaline. 

If hypo crystals effioresce, or become covered with a white 
powder when left exposed to the air, it is a sign that they con- 
tain Glauber’s salt (sulphate of soda). If such be the case, a 
larger quantity of the hypo must be used in making up the 
fixing solution, as Glauber’s salt has no power as a fixing agent. 
Hypo does not keep well in solution, and light hastens its decom- 
position. It is well, therefore, to paste brown paper round the 
store bottles, and to make up fresh solutions every two or three 
weeks. The liypo which has been used for fixing prints may be 
used afterwards, if its strength is not exhausted, for fixing 
negatives, but not vice versa. 

In 1866* Mr. John Spiller recommended the addition of a 
little carbonate of ammonia to the hypo fixing bath, which he 
thought would “ serve a useful end firstly, by aiding the 
hyposulphite in the more perfect removal of the silver; and, 
secondly, by rendering more permanent the double soda and 
silver salt so formed.” About half an ounce of the ammonium 
carbonate should be ground to powder and added to each quart 
of the hypo solution. The prints, after washing, should be 
sponged in order to remove a very slight whitish veil, which 
sometimes results from this employment of the carbonate. 


* British Journal of Photography for June 15, 1866. 


398 


THE CHEMISTRY OF PHOTOGRAPHY. 


There is no doubt but that in the case of albumenized sen- 
sitized paper a small portion of the silver enters into combina- 
tion with the albumen to form a complex compound called 
albuminate of silver ; a compound which it is difficult, if not 
impossible, to remove, and which probably contributes to the 
fading of such prints. 

As far back as 1862 we find that well-known chemist, Mr. 
John Spiller, writing * : Proceeding in the next place to 

inquire into the disposition of the silver and gold upon the 
surface of albumenized proofs, I have been somewhat sur- 
prised to find so much silver existing in the sky and other 
perfectly protected portions of the print, f an observation 
which led me to examine a large number of photographs, and 
cuttings removed from the same, preliminary to mounting, 
also the productions of other operators besides the work 
executed by our own department ; and in no instance have I 
failed to detect silver by the discoloration on moistening the 
albumenized surface with sulphide of ammonia, and allowing 
this reagent to dry upon the paper. But if prints upon plain 
paper be similarly tested there is no evidence of any silver 
remaining in the white parts of the picture, nor on the back 
or unprepared side of albumenized prints will any silver be 
found, a conclusive proof that the same treatment which effec- 
tually removes the whole of the silver from plain paper in the 
course of fixing and washing, is not capable of dissolving out 
entirely the silver from albumenized surfaces. 

As a confirmatory experiment, however, I sensitized plain 
salted paper and three different samples of albumenized paper 
in the same nitrate of silver solution, and, as soon as dry, they 
were, without exposure to light, all washed together in several 
changes of common water, then fixed in a newly-made solu- 
tion of hyposulphite of soda (one ounce of the crystals to four 
ounces of water) and again repeatedly washed as usual, until, 
after a twenty-four hours’ interval and a plentiful supply of 
water, they were judged to have been sufficiently washed. 
When dry, the sheets of albumenized paper contained silver 


* Photographic News for October 3, 1862, p. 471, 

t See also Carey Lea in British Journal of Photography for July 27, 1866. 


THE CHEMISTRY OF ‘‘FIXING” PROCESSES. 399 

in quantity sufficient to give a dark stain with sulphide of 
ammonium, whilst the plain paper did not contain a trace. 

“Since making this observation I have endeavored to find 
some ready means of separating this last portion of metal from 
its combination with albumen, and have subjected the sample 
of paper to treatment with hot and cold salt brine, tartaric acid, 
the tartrates, and a variety of other salts, without any appre- 
ciable effect ; a second immersion in hyposulphite of soda 
removes some of the silver ; and iodide of potassium, and the 
citrates appear to dissolve out a larger proportion, but I am 
still in search of a solvent which is at once both cheap and 
efficient. 

“As to the disadvantages arising from the existence of 
silver in the pure whites of the photograph, it must be remem- 
bered that they are always liable to discoloration by exposure 
to an impure atmosphere, and likewise by the effect of sulphur 
contained in the albumen itself as a constituent, which, if 
liberated by incipient decomposition or other cause, would 
immediately unite with the silver, and give rise to those yellow 
apjiearances so commonly observed in the early stages of 
fading.” 

In the same periodical for 18^4 (page 22), Mr. H. Mathe- 
son stated that he had prevented discoloration of the whites 
of albumenized prints by soaking them, after toning, in a 
“ solution of twenty grains of potassium iodide in about a pint 
of water for five minutes,” and then fixed them in hypo as 
usual. 

Replying to this on February 12, 1864 (page 74), Mr. 
Spiller states that although such treatment might be effective 
in removing any traces of “free nitrate,” yet it could not 
remove “ that small proportion of silver which always enters 
into chemical combination with the albumen, and is not after- 
wards soluble in the hyposulphite of soda.” 

Gaudin Introduces Fixing with Cyanide of Potassium. 

Potassium cyanide as a fixing agent was used mainly in 
connection with the positive wet-collodion process. The first 
reference I can find to it is in the French periodical. La 


400 


THE CHEMISTRY OF PHOTOGRAPHY. 


Lumiere^ for April 23, 1853, where M. Gaudin recommends it 
strongly as being “more convenient and more economical” 
than hypo. It was used in the proportion of eight or ten 
grains to the ounce of water, and was poured upon the devel- 
oped wet-collodion plate while the latter was held level by one 
corner. The solvent power of potassium cyanide upon silver 
iodide is very great, and in a minute or two the picture w^as 
fixed. It gave a bright, clean, and vigorous picture, whiter 
and with more detail than hypo, and was a great favorite in 
consequence during the reign of the “ambrotype” (as the 
positive collodion photograph on glass was called) between 
1853 and 1858. It still lingers, however, among the ferro- 
type workers, who find it useful because it does its work so 
quickly ; a great advantage to this class, since being workers 
by the wayside, on the beach, etc., they desire to complete 
and deliver their pictures in a few minutes. 

The chemical action is as follows : 

Agl + 2KCN = AgK(CN)3 

Silver Iodide and Potassium Cyanide produce Potassium Silver Cyanide 

• + KI 

and Potassium Iodide. 

The potassio-silver cyanide is one of the so-called “double 
salts,” and the important difierence between it and the silver 
cyanide is that the former is very soluble in w^ater, while the 
latter is not. Thus it is easy to wash the potassio-silver 
cyanide out of the film and off the plate. 

But potassium cyanide is so powerful that it is able even to 
dissolve a little of the metallic silver forming the image, when 
this is in a very finely divided state.'^ In the case of the 
“ambrotypes” the deposit of silver was coarse-grained ; but 
in the ordinary negative collodion process, and in the calotype 
process on paper, the deposit of silver is much finer, and 
when potassium cyanide is used as a fixing agent the half- 
tones suffer. In the case of silver bromide, and of silver 
chloride, the action of the cyanide is even greater, so that it is 
never used to fix pictures made with them. The potassium 


* Carey Lea, in British Journal of Photography for September 7, 1866. 


THE CHEMISTRY OF “ FIXING ” PROCESSES. 401 

cyanide of commerce is, moreover, always alkaline — owing to 
the presence of carbonate of potassium as an impurity — and 
it has consequently a tendency to soften the gelatine film. 
For over-printed silvei prints, however, potassium cyanide is 
sometimes useful as a reducer. The prints are immersed in 
an extremely weak solution (about one grain of cyanide to 
half a gallon of water) and in the course of an hour or two 
much of their superfluity of silver will be removed. 

Snelling, in his “Dictionary of the Photographic Art” 
(New York, 1854), says : 

“Cyanide of potassium is excellent, in solution, for remov- 
ing — with the aid of a sable pencil — the black spots which so 
often spoil a good proof. The operator, however, should be 
careful to arrest its action at the proper moment, as if left too 
long it will remove too much. To do this you must wash the 
proof in clear water acidulated with prussic acid, and again 
wash it in several waters. * It is also used for removing 
stains of nitrate of silver from the hands in the proportion of 
one grain of the salt to ten grains of water.” 

Snelling adds that “ Cyanide of potassium dissolves the 
iodide, chloride and bromide of silver,” but he does not speak 
of it as a fixing agent, so that presumably it was little if at all 
employed at that time in the United States for such a purpose. 
Indeed, Snelling says elsewhere in his bcok : “Hyposulphite 
of soda is the best fixer that can be employed, as well for 
negative as for positive proofs.” 

A patent taken out in England by “ Peter Armand le 
Comte de Fontaine Moreau,” on December 13, 1854, directs 
the developed picture (a collodion positive) to be “ washed 
several times in fresh water, and then plunged into a bath 
composed of cyanide of potassium and distilled water” in 
order to fix it. 

The extremely poisonous nature of potassium cyanide is a 
great drawback to its general usefulness. It is, in itself, 
highly poisonous ; but if any acid be added to it, fumes of 
hydrocyanic (prussic) acid are given off, the inhalation of 
which is usually followed by fainting and illness, and not un- 
frequently by death. 


402 


THE CHEMISTEY OF PHOTOGEAPHY. 


Many photographers have committed suicide by swallowing 
this deadly poison, three grains of which is a fatal dose ; it lay 
ready to their hand, and they knew that it produced a rapid 
and comparatively painless death. It is better banished alto- 
gether from the shelves of the ordinary worker. The symp- 
toms of poisoning by cyanide are “insensibility, slow, gasping 
respiration, dilated pupils, and spasmodic closure of the jaws.” 
There is no certain remedy, although the flowing of a stream 
of cold water over the head and neck has been found useful. 
If the cyanide touches sore places or abrasions in the skin it 
produces a painful smart, which may be eased by the early 
application of sulphate of iron. 

Watee as a Fixing Agent. 

The ideal fixing process — that in which the prints require 
nothing more than a good washing in plain water — at present 
belongs to one printing process only, that known as cyanotype, 
or the “ blue ” process. 

The two salts with which the paper is coated are potassium 
ferricyanide and ammoiiio-citrate of iron, each of them soluble 
in water. By exposure to the light these two substances are 
caused to combine, when they form an insoluble blue com- 
pound closely allied to “ Prussian blue.” 

The excess of soluble matter is then removed by simple 
washing in v^ater, and the picture is then seen in white lines 
on a blue ground. 

Hydeochloeic Acid as a Fixing Agent. 

In the “ platinotype ” printing process we coat paper with 
ferric oxalate (wdiich is converted into ferrous oxalate by the 
action of light) and chloro-platinite of potassium. The pic- 
ture is ultimately produced (by floating the exposed print upon 
a bath of hot oxalate of potash) in metallic platinum. The 
iron, etc., salts which remain are soluble in a weak solution of 
hydrochloric acid (1 to 60), and the prints are allowed to soak 
in this for a few minutes until their yellow hue (due to the 
iron) has completely disappeared. Finally they are well 
washed for half an hour in plain water to remove the acid. 


THE CHEMISTEY OF FIXING ” PROCESSES. 403 

The Acid Sulphite Fixing Bath. 

In the early part of the year 1889 several articles appeared 
in the German photographic papers {Photographische Corre- 
spondenzen^ Archiv^ etc.), principally from the pen of Dr. 
Alexander Lainer, recommending the addition of acid sodium 
sulphite to the ordinary hypo bath for fixing negatives. The 
advantages were said to be that the plates were both fixed and 
cleared^ and that the fixing bath remained clear and in good 
order for a longer period. 

Sodium sulphite (Nag SO 3) has long been used to keep the 
pyro solution clear during development ; and acid sodium sul- 
phite (Na HSO3) can be made by adding an acid — preferably 
tartaric acid — to the solution of the sulphite. 

Lainer recommends that the acid fixing bath be made up as 
follows : 

1. Make up thirty-four ounces of an ordinary hypo bath, 
strength one ounce of hypo to four ounces of water. 

2, Make solutions of tartaric acid and of sulphite of soda, 
each of the same strength (1 to 4). 

Mix one ounce of the tartaric-acid solution with two and a 
half ounces of the sulphite solution ; shake well and then add 
the mixture to the thirty-four ounces of hypo solution. Hy- 
drochloric acid may be used instead of tartaric. 

The acid sulphite is also sold commercially as a lye, in 
which state it is a pale yellowish liquid, smelling strongly of 
sulphurous oxide gas. With this it is only necessary to add 
two ounces of the lye to each quart of hypo solution. Besides 
the advantage already named, this ‘^acid fixer” fixes very 
rapidly. 

The use of the acid in the bath is really to liberate sulphu- 
rous acid (SOg), a substance which has powerful decolorizing 
properties. This acid was recommended in 1885'^ for use in 
the clearing bath, instead of the citric or hydrochloric acid as 
usually employed. In December, 1887, the late Mr. H. B. 
Berkeley patented a combination of hypo and free sulphurous 
acid as a fixing agent. Sulphurous acid, unlike most other 


* British Journal of Photography for June, 1885 (editorial). 


404 


THE CHEMISTRY OF PHOTOOEAPHY. 


acids, does not decompose lijpo, and therefore the liberation 
of sulphur, with its baneful effect upon the negative, is not to- 
be feared. 

Why do Photographs Fade ? 

The early photographers took bat little pains to get rid of 
the excess of hypo with which their prints were impregnated. 
It is true that in the case of collodion positives a good washing 
and rinsing will remove the hypo from the porous and inert 
collodion in a few minutes, but in the case of paper the hypo 
clings more obstinately to the fibres. 

About the year 1854 the general fading of paper prints had 
become so evident, that the Photographic Society of London 
appointed a committee to take into consideration the ques- 
tion of the Fading of Positive Photographic Pictures upon 
Paper.” The report of this committee is published in the 
Society’s journal for 14ovember 21, 1855, and the following 
eminent names are appended to it : Delamotte, Dr. Dia- 

mond, T. F. Hardwich, Malone, John Percy, H. Pollock, Geo. 
Shadbolt. 

The report is so important — and withal so compact — that we 
reproduce it in f ull : 

The Committee, in this Deport, propose to confine them- 
selves to a statement of the evidence which they have collected 
as to the permanence of photographs up to the time of their 
appointment, adding some facts in connection with the causes 
of fading, which are of practical value, reserving for a future 
occasion the scientific part of the investigation. 

Evidence of Permanence . — The Committee have unques- 
tionable evidence of the existence of photographs which have 
remained unaltered for more than ten years, prepared by salt- 
ing plain paper witli a chloride, afterwards making it sensitive 
with either nitrate or ammonio-hitrafe of silver, mixing with a 
freshly made solution of hyposulphite of soda and washing in 
water ; also of positivesf produced by Mr. Talbot’s negative 
process. 


* That is, since 1845. W. J. H. 
f i.e.^ developed prints. W. J. H. 


THE CHEMISTRY OF FlXmO ” PROCESSES. 405 

They have not been able to obtain evidence of photo- 
graphs having been prepared at all upon albumenized paper,* 
or colored with a salt of gold or fixed with ^ old hypo,’ so 
long ago as ten years. 

^^They have, however, ample evidence of the existence 
of unaltered photographs so prepared, five, six, or seven 
3^ears ago. 

They have not found that any method of printing which 
has been commonly followed, will necessarily produce fading 
pictures, if certain precautions be adopted ; nor have they evi- 
dence that any method which has been adopted, will not 
produce fading pictures unless such precautions are taken. 

Causes of Fading . — The most common cause of fading 
has been the presence of hyposulphite of soda, left in the paper 
from imperfect washing after fixing. 

“ The committee think it right to state, that they have been 
unable to find any test to be relied upon, which can be used to 
detect a minute portion of hyposulphite of soda, in the 
presence of the other substances which are obtained by boiling 
photographs in distilled water and evaporating to dryness ; yet 
they have no doubt of the truth of the above statement, from 
the history given of the mode of washing adopted. 

‘‘ The continued action of sulphuretted hydrogen and water 
will rapidly destroy every kind of photograph ; and as there 
are traces of this gas at all times present in the atmosphere, 
and occasionally in a London atmosphere very evident traces, 
it appears reasonable to suppose that what is effected rapidly 
in the laboratory with a strong solution of the gas, will take 
place also slowly but surely in the presence of moisture, by the 
action of the very minute portion in the atmosphere. 

“ The committee find that there is no known method of pro- 
ducing pictures which will remain unaltered under the con- 
tinued action of moisture and ^he atmosphere in London. 

‘‘ They find that pictures ma}" be exposed to dry sulphuretted 
hydrogen gas for some time with comparatively little altera- 


* Such paper, coated with white of egg to give it a gloss, and to prevent the 
silver salt from sinking too deeply into the paper, was introduced about 1852. The use of 
chloride of gold for toning paper prints became general about the same time. W. J. H. 


4:06 


THE CHEMISTRY OF PHOTOOEAPHY. 


tion, and that pictures, in the coloration of which gold has 
been used, are acted upon by the gas, whether dry or in solu- 
tion, less rapidly than any others. 

They also find that some pictures which have remained 
unaltered for years, kept in dry places, have rapidly faded 
when exposed to a moist atmosphere. 

Hence it appears that the most ordinary cause of fading 
may be traced to the presence of sulphur, the source of which 
may be intrinsic from hyposulphite left in the print, or ex- 
trinsic from the atmosphere, and in either case the action is 
much more rapid in the presence of moisture. 

Mode of Mounting Photographs . — The committee find 
that taking equal weights, dried at a temperature of 212 deg. 
of the three substances most frequently used, viz : gelatine, 
gum, and paste, the latter attracts nearly twice as much mois- 
ture as either of the former ; and as in practice a much smaller 
weight of gelatine is used than of gum, gelatine appears to be 
the best medium of these three ; and the Committee have evi- 
dence of fading having in some cases been produced by the use 
of paste. 

“In illustration of some of the circumstances alluded to 
above, the Committee think it well to mention some instances 
of prints at present in their possession. 

“ Out of several prepared together in 1844, three only are 
unaltered, and these were varnished soon after their prepara- 
tion with copal varnish. 

“ Half of another print of the same date was varnished, and 
the other half left ; the unvarnished half has faded, the var- 
nished remains unaltered. 

“ Three pictures were prepared in 1846, all at the same time, 
with the same treatment ; when finished, one was kept un- 
mounted ; the other two were mounted with fiour-paste at the 
same time, one of these latter having been first coated with 
Canada balsam ; at present the unmounted one and the one 
protected with balsam are unchanged, whereas the other has 
faded. 

“A picture prepared in 1846 was so exposed that the lower 
part of it became wetted with rain ; at present the part 


THE CHEMISTKY OF “ FIXING ” PROCESSES. 


407 


so wetted has faded, while the rest of it remains unaltered. 
Several pictures were prepared and mounted about ten 
years ago, and kept in a dry room for about three years 
without any change, after which they were placed in a very 
damp situation, and then faded decidedly in a few months. 

The Committee propose very shortly to actually test the 
durability of the various modes of printing, by exposing pic- 
tures to different treatment, and they have been fortunate 
enough to obtain a grant of space for this purpose from the 
Crystal Palace Company. 

^‘The Committee make the following suggestions, arising 
out of the above report : 

“1. That the greatest care should be bestowed^ upon the 
washing of prints after the use of hyposulphite of soda, and 
for this purpose hot water is very much better than cold. 

2. The majority of the Committee think that gold, in 
some form, should be used in the preparation of pictures, 
although every variety of tint may be obtained without it. 

“ 3. That photographs be kept dry. 

‘^4. That trials be made of substances likely to protect the 
prints from air and moisture, such as caoutchouc, gutta percha, 
wax and the different varnishes.” 

The simple conclusions arrived at in this able report are as 
true to-day as they were thirty-eight years ago. Of course 
they apply — as the report applied — to ordinary silver prints 
only. Hypo left in the print ; plus sulphuretted hydrogen 
(from burning gas-jets, etc.), and moisture in the air ; these 
are the great enemies of silver prints; and they combine — 
sooner or later — to reduce the brilliant silver print to a yellow 
faded shadow of its former self. Sulphur is liberated from 
either hypo or sulphuretted hydrogen, and it combines with 
the black silver which forms the picture to produce the yellow 
or yellowish-brown compound known as sulphide of silver 
(Ag,S). 

We have, it is to be feared, not arrived much nearer the 
solution of a ‘^permanent printed-out silver print on albumen- 
. ized paper ” since the report quoted above was prepared in 
1855. The remedjq to the more earnest workers of the pres- 


408 THE CHEMISTRY OF PHOTOGRAPHY. 

eut age, seems to be to discard tliis printing process altogether. 
The meretricious gloss of the paper is as inartistic as the pic- 
tures printed upon it are fleeting. 

Prints on matt-surface paper seem to be somewhat more 
permanent than those on albumenized paper, the reason being 
that albumen itself contains a little sulphur (witness the 
blackening of our silver egg-spoons) and is a substance very 
liable to decomposition ; while the developed prints on bro- 
mide paper are certainly far more lasting. But the prints 
produced by the platinotype ” process, and by the “ carbon ’’ 
process, are the only ones which can be guaranteed with cer- 
tainty to be as permanent as engravings. 

However, much might be done to lengthen the lives of 
silver prints, and to give them a fair chance.” To remove 
the hypo thoroughly the prints ought to be frequently 
as well as washed in many changes of water. Capt. Abney 
uses a sponge,* and with it presses the print upon a glass 
plate some ten or twelve times, allowing the print to soak in 
fresh water for ten minutes between each squeezing. It is 
also an excellent plan to soak the prints alternately in hot and 
cold water. 

Then the cardboard on which the print is mounted ought 
to be free from injurious chemicals (hypo itself is much used 
in the manufacture of paper and cardboard) ; the mountant 
should be suitable (as starch, or gelatine) and freshly made ; 
and, lastly, the picture should be protected from the air by 
being well framed, or by means of a good album. 

Still, “give up the ordinary printing process on (silver) 
sensitized paper ” is our advice to all who desire their work 
to “live after them.” 


* A soft roller squeegee acts as well as or even better than a sponge. 


CHAPTEE XXXIY. 


THE CHEMISTRY OF HYPO ELIMINATORS. 

Attempts to Remove Hypo by Chemical Means. 

The length of time — twenty-four hoars, according to many 
writers — required to remove the last traces of hypo from prints 
by the action of water alone, and the physical exertion needed 
when such a plan as sponging or pressing the prints is adopted, 
have led to many attempts to find some speedy and effective 
means of removing this treacherous fixing agent from the 
paper. In effecting the removal or destroyal of hypo these 
plans are for the most part effective ; but they are, unfortu- 
nately, apt to introduce at the same time other chemical com- 
pounds whose presence may be even more harmful. 

Peroxide of Hydrogen . — At a meeting of the Photographic 
Society of Scotland on May 8, 1866, a paper sent by Dr. 
Angus Smith, of Manchester, was read,* in which he recom- 
mended a solution composed of one part of the liquid perox- 
ide (as sold commercially) to one thousand parts of water as a 
means of oxidizing the hyposulphites remaining in the prints 
into ‘Hnnoxious and harmless sulphates.” Peroxide of hydro- 
gen (Hg Og) was then sold at five shillings per pound, but it 
is now (1892) only two shillings per pound. It is a substance 
which is very unstable, and in the presence of the other chem- 
icals it decomposes into water and oxygen as follows : 

Ho O2 o -f HgO 

Hydrogen Peroxide produces Oxygen and Water. 

The oxygen then combines with the hypo : 

NagSgOg -h 2 O 2 4- H 2 O = 2 NaHS 04 

Sodium Hypo- and Oxygen and Water produce Sodium Hydrogen 
sulphite Sulphate. 

* See British Journal of Photography for 1866, pp. 226, 232, 267, 316, 327. 


410 


THE CHEMISTRY OF PHOTOGRAPHY. 


The sodium -hydrogen sulphate is readily removed from the 
paper by a short washing in water ; but even if any be left in 
the print it would be comparatively harmless. 

The ^objections to the use of hydrogen peroxide as a hypo 
eliminator are that it is liable to destroy the more delicate half- 
tones of the image, and that it does not keep well. Where 
hydrogen peroxide cannot be bought, it may be readily pre- 
pared by mixing one ounce of glacial acetic acid with four 
ounces of cold water and adding one ounce of powdered barium 
dioxide. The prints should be immersed in this solution for 
five minutes. 

Sodiaim and Other Hypochlorites as Eliminating Agents. 
— In 1864 Mr. F. W. Hart told the members of the South 
London Photographic Society (see British Joicrnal of Pho- 
tography for March 1, 1864) that “with the desire, if possible, 
to secure the permanency of silver prints, he had experiment- 
ed as follows : Two prints were taken (for which the paper 
had been prepared in the usual way) and treated throughout 
in an exactly similar manner up to the point of toning, when 
one, after being toned, was immersed in hyposulphite of soda, 
rinsed, and then immersed in an aqueous solution of chlorine 
and chloride of barium. The effect of this treatment he (Mr. 
Hart) had found to be the conversion of any remaining traces 
of hyposulphite of soda into sulphate of barium and chloride 
of sodium, thereby insuring the non-existence of sulphur in 
the prints.” 

We cannot hear of this plan having been adopted by any 
one. The free chlorine would probably affect the half-tones 
of the prints considerably. 

But in 1866 the same worker (Mr. F. W. Hart) proposed a 
method which has been more generally tried, though it cannot 
be said to have been adopted to any extent. Mr. Hart’s sec- 
ond proposal, read as a paper before the South London Photo- 
graphic Society (see British Journal of Photography for 
June 22, 1866; see also editorial article in number for June 
29, 1866), was on the subject of “The Elimination of the 
Double Hyposulphites of Soda and Silver from Photographic 
Prints,” The substance now proposed to be used for this pur- 


THE CHEMISTRY OF HYPO ELIMINATORS. 411 

pose was hypochlorite of soda. When this is brought into 
contact with hypo the following reaction occurs : 

4NaC10 + NasSgOg + = 4NaCl ^ + 

Sodium and Sodium and Water produce Sodium and 

Hypochlorite Hyposulphite Chloride 

2 NaHS 04 

Sodium Hydrogen Sulphate. 

Sodium hypochlorite is sold commercially as Labarraques’ 
solution.” It can readily be prepared by dissolving a quarter- 
pound of carbonate of soda in ten ounces of water, and two 
ounces of chloride of lime in thirty ounces of water. Mix, 
boil, and hlter. 

Other hypochlorites have since been introduced; and in 
America, zinc hypochlorite, sold as “ Flandreau’s eliminator,” 
has been rather a favorite. Its chemical action is as follows : 

2 ZnCl 202 -f Na 2 S 20 g 4 - H 2 O = 

Zinc Hypochlorite and Sodium Hyposulphite a7td Water produce 
2 ZnCl 2 + 2NaHSO 4 

Zinc Chloride aiid Sodium Hydrogen Sulphate. 

If this were all that could happen, then zinc hypochlorite 
and the other hypochlorites would be good hypo eliminators ; 
but they do not keep well, and they are very liable to liberate 
free chlorine, the following secondary reaction then occurring : 

Na2S203 -f 8C1 -I- 5 H 2 O = 

Sodium Hyposulphite and Chlorine and Water produce 
8HC1 + 2NaHS04 

Hydrochloric Acid and Sodium Hydrogen Sulphate. 

The hydrochloric acid so formed would immediately react 
upon more hypo in the following way : 

NagSgOg + 2HC1 = S + SOg 4 - 

Sodium and Hydrochloric prodtice Sulphur and Sulphur and 
Hyposulphite Acid Dioxide 

2NaCl 4 - HgO 

Sodium Chloride and Water. 

JSTow sulphur is perhaps the most injurious substance we can 
possibly have in the print. 


* British Journal of Photography for October 26, 1886. 


412 


THE CHEMISTRY OF PHOTOCEAPHY. 


Potash hypochlorite (commercially known as eau de javelle) 
has also been much used as a hypo eliminator. It is easily 
made by dissolving a quarter of a pound of carbonate of 
potaslf in thirty ounces of water ; then mix two ounces of 
chloride (properly hypochlorite) of lime in ten ounces of 
water. Mix the two liquids, boil, and filter. The chemical 
action is : 

Na^SgOg + 4KC10 + HgO = 

Sodium Hyposulphite and Potassium Hypochlorite and Water produce 
4KC1 + 2NaHS04 

Potassium Chloride and Sodium Hydrogen Sulphate. 

It will be seen that all these hypochlorites are of an unstable 
nature, readily parting with their oxygen and becoming re- 
duced to chlorides. The liberated oxygen combines with the 
hypo to form a sulphate, which is a stable and comparatively 
harmless compound. 

Iodine as a Hypo Eliminator. — Dr. H. W. Yogel was, we 
believe, the first to suggest the use of the elementary body, 
iodine, as an aid to the removal of hypo from prints. Iodine 
is dissolved in a strong solution of potassium iodide until a 
very dark-colored liquid is obtained. After careful washing 
the prints are placed in water to which enough of the iodine 
solution has been added to give it a sherry color. Here the 
prints take a faint blue color. They are then rinsed in a very 
weak solution of mixed sulphite and carbonate of soda (by 
which the blue color is taken out) and are finally well washed 
in water. 

The chemical action of the iodine is as follows : 

2 Na 2 S 203 21 = 2NaI + NagS^Oe 

Sodium Hypo- and Iodine produce Sodium and Sodium tetra- 
sulphite Iodide thionate. 

The two sodium salts formed (the iodide and tetrathionate) 
are very soluble and are easily washed out of the paper. Mr. 
0. B. Lloyd has strongly objected to this method* on the 
ground that sodium tetrathionate is a salt containing much 


* British Journal of Photography for 1887, p. 724. 


THE CHEMISTRY OF HYPO ELIMINATORS. 


413 


sulphur, and that it is readily decomposed. He also thinks 
that the whites of the picture are degraded by the temporary 
dyeing with iodine. 

Alum as a Hypo Remover. — The first reference I can find 
to the use of alum as a hypo eliminator is in the Journal of 
the P hotograpliic Society for the 21st of June, 1855, where 
Sir W. J. Hewton recommends prints to be treated as follows : 
“ Immerse in hyposulphite for about two or three minutes, 
then in alum-water for half an hour, and change the water 
entirely two or three times.” deferring to this note, Mr. T. 
Sutton remarks in the same Journal for August 21: “The 
alum bath recommended by Sir William Aewton is also a use- 
ful suggestion. The alum forms with the hypo a double salt 
(soda-alum), wdiich is highly soluble in water, and I imagine 
comparatively innocent.” 

If this advice was largely acted upon it is not surjDrising 
that very few photographs printed in 1855 are in existence 
in 1890. 

But an antidote to this “ alum ” method was soon supplied 
by T. F. Hardwich, the leading photographic chemist of that 
day ; in the Journal for September 21, of the same year, he 
writes : “ With reference to the use of alum in w^ashing paper 
positives, may I be allowed, on chemical grounds, to raise an 
objection. It is an acid salt^ the sulphuric acid being only 
imperfectly neutralized by alumina, which is a feeble base ; 
hence on mixing it with hyposulphite of soda, sulj^hate of 
soda, sulphurous acid, and sulphur are formed, the reaction 
being the same to all appearance as that of the acids generally, 
upon hyposulphite of soda.” 

Hardwich wrote further upon the subject in the Journal 
for March 21, 1856, wdth the result of convincing ISTewton 
that alum did more harm than good. 

Holmes'’ Ozone Bleeicli. This is a commercial preparation, 
the chemical composition of which has not been published. 
But as its name tells us that it is a product of ozone w^e may 
say that that substance is a condensed form of oxygen, each 
molecule containing three atoms of oxygen (O3), wFile in ordi- 
nary oxygen the molecules consist of two atoms only (Og). 


414 


THE CHEMISTRY OF PHOTOGRAPHY. 


Ozone is an active oxidizing agent, and it would convert hypo- 
sulphite of soda into sulphate, thus : 

NasS.Og + 2O3 + HgO = 2 Na HSO4 + 

Sodium Hypo- and Ozone and Water produce Sodium Hydro- and 

sulphite gen Sulphate 

O. 

Oxygen. 

But the ozone is very likely to attack the delicate half-tones 
of the prints. 

^Yater the hest Hyjpo Eliminator. Having described the 
various means which have been suggested for getting rid of 
hypo, chemically, in a short time, we must own to having little 
belief in any of them. The so-called eliminators — while 
doubtless removing the dreaded hypo, as hypo — frequently 
form new compounds which may be no less dangerous. They 
generally damage the print in some way or other, perhaps 
destroying the more delicate half-tones, or injuring the purity 
of the whites. 

Supposing the print to have been properly fixed in two 
baths of fresh hypo; then nothing can excel the repeated 
washings in changes of warm and cold water — combined with 
pressure — which we have recommended. 

We can best conclude this part of our subject by using the 
words of the writer of an editorial article in a recent number 
of the British Journal of Photography “And what we 
desire to impress upon all is, that the same amount of care 
applied in simple washing will effect the purpose in view, the 
removal of the hyposulphites at least, as efficiently as any 
eliminator, and without any danger.” 

How to Detect the Presence of ^PlypJ'^ in Plates or Prints. 
— It is useful to have some means of determining whether we 
have been successful in our endeavors to wash our negatives 
and prints thoroughly. The following tests will indicate 
whether any hypo still remains in them : 

The Permanganate Test. — Dissolve two grains of potassium 
permanganate and twenty grains of potassium carbonate in 
one quart of distilled water. This solution is of a fine pink 


* For 18th October, 1890, 


THE CHEMISTRY OF HYPO ELIMINATORS. 415 

color. Take the water in which the negatives or prints have 
last been soaking for ten minutes or more, and pour it into a 
clean glass bottle, which will hold, say, one pint. To this 
clear water add five or ten drops of the pink (permanganate) 
solution. If the water be pure it will assume a pale pink 
tinge ; but if any hypo be present the color will change to a 
light shade of green. The bottle should be shaken well, and 
allowed to stand for ten minutes. 

The Starch Iodide Test. — Powder and boil a piece of starch 
the size of a pea in quarter of an ounce of water until a clear 
solution is attained. Add to this one drop of tincture of 
iodine (iodine dissolved in alcohol) which will produce a dark- 
blue color. Fill one test-tube Avith distilled water, and an- 
other with the water to be tested for the presence of hypo. 
Add to each test-tube one drop of the blue solution. If any 
hypo be present the blue color will disappear. The tubes 
should be shaken well, gently warmed, and examined side by 
side in front of a piece of Avhite paper. 

Hypo in Prints. — The paper used for printing photographs 
upon is all but invariably sized with starch. Make an 
extremely weak solution’^ of potassium iodide, and apply it 
with a brush to the back of the print to be tested. A blue 
color will indicate the absence of hyj)o. 

An Electrical Test for Hypo. — In 1866 Dr. Deissig, of 
Darmstadt, used f a test which showed that the amount of 
sulphur was very large,” in several faded prints examined by 
means of it. The prints were soaked in water, and two strips 
of polished silver, connected by wires with a single galvanic 
cell, were then dipped into the solution. The presence of 
sulphur was indicated by a black stain upon one of the silver 
plates. Keissig patented this process in England (March 10, 
1865). 

Ro\igh Test for Hypo. — If the amount of hypo remaining 
in a print or in a negative be at all large it may be detected 
by allowing the last few drops which will fall from either 
when drained, to drop into the mouth. Mention has already 


* Two grains of the salt in a pint of water, 
t British Journal of Photography ^ p. 83’i. 


416 


THE CHEMISTRY OF PHOTOGRAPHY. 


been made of the intense sweetness of the double salt of soda 
and silver which the hypo forms, and which it is our object to 
remove from our negatives and prints. The absence of any 
sweet taste would, however, only indicate that the greater jpart 
of the dangerous salt had certainly been removed. 

Nitrate of Silmr Test . — Dr. Bannon * finds that silver 
nitrate is a delicate test for discovering traces of hypo in 
prints or films. The water from the prints, etc., should be 
allowed to drain into a test-tube and heated, and then a few 
drops of silver nitrate solution added to it. A black precipi- 
tate will be formed if the one ten-thousandth part of hypo be 
present ; while a still smaller amount will give a yellow pre- 
■ cipitate. 

Literature of Fixing Processes, Etc. 

In addition to the references given in the course of these 
articles on The Chemistry of Fixing,” we append a list of a 
few papers, etc., written during the last few years, which 
have been consulted upon the subject. 

From The Photographic Times. 

The Acid Fixing Bath, Etc. (Editorial), p. 171, for 1890. 

Lainer., A . — An Acid Fixing Bath without Turbidness, p. 
238, for 1889. 

Fading of Silver Prints, p. 540, for 1889. 

Broekway^ G. M. — Faded Prints, p. 533, for 1888. 

Alum as a Hypo Eliminator (Editorial), p. 79, for 1887. 

Hypo Eliminators ; Do they Eliminate ? (Editorial), p. 232? 
for 1887. 

Sherman, W. L/.— Hypo Eliminator, pp. 292, 359, 579, 660, 
for 1887. 

From The British Journal of Photography. 

The Fading of Prints (Editorial), p. 177, for 1890. 

Starnes, II. S . — The Probable Permanence of Plain Paper 
Prints, p. 85, for 1890. 


'^'Photographic Times., p, 38, for 1889. 


THE CHEMISTRY OF HYPO ELIMINATORS. 


417 


Acid Solutions of Hjpo (Editorial), p. 50, for 1890. 

The Acid Fixing Bath, p. 33, for 1890. 

A Substitute (Magnesium Chloride) for Hjpo as a Fixing 
Agent (Editorial), p. 210, for 1890. 

Ultimate Effects of Hypo Eliminators (Editorial), p. 225, 
for 1890. 

Permanency of Bromide Paper Pictures (Editorial;, p. 289, 
for 1890. 

Dunmore^ E. — About Hypo, p. 327, for 1890. 

Burton^ W. K. — A Method of Bapidly Eliminating Hypo 
from Silver Prints, and other Notes on Silver Printing, 
p. 233, for 1889. 

Hypo and Hypo Solutions, pp. 360 and 389, for 1889. 

Sulphurous Acid in the Fixing Bath, p. 806, for 1889. 

Dangerous Hypo Eliminators, p. 678, for 1889. 

Hart^ F. TF. — The Early History of Hypo Eliminators, p. 
151, for 1888. 

The Bestoration of Faded Photographs, p. 337, for 1888. 

Fixing Bromide Prints — A Caution, p. 709, for 1888. 

Dmies^ W. II. — Aids to the Preservation of Photographic 
Prints, p. 759, for 1888. 

Starnes^ II. S. — Permanence of Photographic Prints, pp. 
24, 70, 101, for 1888. 

Elliott^ Dr. A. II. — A Search for a Substitute for Hypo, 
pp. 639, 554, for 1887. (Bead before American Conven- 
tion.) 

Dawson,^ Geo . — On the Fading of Silver Photographs, pp. 
552, 600, 616, for 1887. 

Harrison., TF. II . — The Fading of Silver Prints, p. 824, for 
1887. 

An Unsuspected Cause of Fading, p. 577, for September 
16, 1887. 

Fixing and Washing Gelatine-Bromide Enlargements, p. 1, 
for January 7, 1887. 

'Pringle., A . — Sulphnration of Bromide and Platinum 
Prints, p. 2, January 7, 1887. 

Berkeley., Mansfield., Etc. — Permanence of Prints, pp. 30, 
45, 59, 87, 101, no, 159. 


418 


THE CHEMISTEY OF PHOTOGEAPHY. 


Lloyd ^ 0. B. — Siilpliuration of Platinum and Silver Prints, 
p. 36, for January 21, 1887. 

Starnes^ H. S. — Sulpliuration of Prints, p. 217, for April 8, 
1887, and p. 360, for June 10, 1887. 

Lloyd^ C. B. — Iodine and Hypo, p. 724, for November 18, 
1887. 

Bridge^ F. A. — Spots, Stains, and Fading, p. 775, for 
Der.ember 9, 1887. 

Bow, R. II. — Fixing Silver Chloride Prints by Means of 
Solution of Ammonia, p. 231, for April 15, 1887. 

Dawson, G. — Fading of Silver Prints, pp. 321, 336, 748, 
for 1886. 

Pringle, A. — Permanence of Prints, pp. 601, 794, 813, for 
1886. 

I eine, R. — How to Prevent Silver Prints from Fading, p. 
732, for November 19, 1886. 

L aidon, TF. K. — Fixing Prints, Etc., ]>. 616, for October 1, 
1886. 

On Hypo Eliminators Generally (Editorial), p. 645, for 
October 15, 1886. 

The Hypochlorites in Photography, p. 661, for October 22, 
1886. 

Permanence of Gelatino-Bromide Pictures, p. 301, for May 
14, 1886. 

Feom The Photogeaphic News. 

Higgins, J. J. — The Fixing Bath, p. 732, for 1891. 

Lainer, A. — A Mixed Alum and Fixing Bath, p. 538, for 
August 16, 1889. 

Barnes, C. B. — The Fading of Silver Prints, and its Cause, 
p. 715, for November 1, 1889. 

Dehenham, W. E. — Fading of Silver Prints, p. 777, for 
November 22, 1889. 

Gunther, II. K. — Eliminating Hypo from Prints by Means 
of Common Salt, p. 355, for May 31, 1889. 

Jones, Chapman. — Iodine as a Hypo Eliminator, p. 795, 
for December 16, 1887. 


THE CHEMISTRY OF HYPO ELIMINATORS. 




Maxims for Fixing. 

1. Use freshly-made hypo strengtl] 1 to 5 for negatives; 
1 to for prints. 

2. Keep the hypo alkaline by the addition of enough 
ammonia to cause it to smell faintly, say a teaspoonful to each 
quart. 

3. Use two fixing baths ; allow each negative (or print) to 
remain at least ten minutes in each of the two baths. 

4. As soon as the first hath begins to fix slowly and become 
dark-colored, throw it away and let the second bath take its 
place. Make up a fresh bath to take the place of bath Ko. 2. 

5. Do not expose negatives (or prints) to white light while 
fixing, or until the hypo has been rinsed ofi. This is espe- 
cially necessary with plates that have not had an alum bath 
before fixing. 

6. After fixing rinse the prints or negatives in several 
changes of water to remove the surface hypo, and then wash 
for six hours in running water. 

7. Fresh hypo solution should be made up for each batch of 
prints. For negatives the same hypo bath may be used over 
and over again, until it begins to work slowly. The bath used 
for fixing prints may afterwards be used for fixing negatives; 
but not vice-versa. After all, hypo is so cheap that it is poor 
economy to attempt to save in it, at the possible risk of the 
permanency of your results. 

8. Wash the prints in three or four waters before you put 
them in the fixing bath. 

9. 2Iove the Prints frequently while in the fixing baths. 
If this is not done, yellow spots and stains may be expected. 

10. Don’t expect that other people — a ‘Aiandy boy,” for 
example — will fix your prints, etc., as carefully as you wmuld 
do it yourself. The old proverb specially holds good in pho- 
tography : “ If you want a thing done well, do it yourself ! ” 



INDEX. 


Scientific names of chemicals are printed in alphabetical order on pages 55 to 58, and 


59 to 163, and are therefore not given in this 
Names of persons are printed iti italics. 


PAGE 

Abney, Capt. .18Q, 189, 201, 223, 

225, 231, 240, 264, 408 

Acetate Toning Bath 344 

Acids 16 

Acid Development 224 

Acid Sulphite 403 

Ackland, W 145 

Air ... 3 

Alabastrine Process 312 

Albuminate of Silver 398 

Albumen on Paper 248 

Albumenized Paper 248, 254 

Albumen Process 171 

Alchemists. . 10 

Alcohol 54 

Alkali 68 

Alkaline Developer 221 

Alkaline Development. .. .214, 224 

Alkaline Toning 340 

Allotropism 178 

Allotropic Silver 192 

Alum to Remove Hypo 413 

Amber 158 

Ajidresen., Dr 122, 138 

Ammonia 215 

Ammonia for Intensifyii g 321 

Ammonio-Nitrate of Silver.... 140 

Ammonia for Fixing 383, 387 

Ammonium Sulphide 323 

Ammonio-Nitrate Bath 253 

Anhydrous 17 

Anhydrides 16, 81 

Aniline Dyes 330 

Anthony, H. T. 214, 215 

“Anti-chlor” 83 

Apparatus 40, 44 

Aqua-regia 53, 120 

Aquafortis 119 

Archer, F. S 85, 172, 211, 312 

Arfvedson 113 

' Aristotype 257 

Atoms 197 

Atomic Theory 19 

Atomic Weights 11 

Auricomus. 105 

Audra, E 285 

Azaline 246 


index. 


PAGE 

I Balance (Chemical) 40 

I Balard 75 

Bannon, Dr 416 

Barnes, C. B 174, 346 

Bases 16 

Basylous 12, 13 

Beaufoy’s Acetic Acid 59 

Beccarius 188 

Becqnerel, E 200, 239, 258 

j Bedding, T 203 

j Bedford, W 159 

I Beiitzki, L 292, 293 

i Bennett 176, 219 

I Berkeley, H. B 153, 403 

i Bert, M. Paul 241 

Bichromate of Potash Reducer. 290 

Bichromate of Potash 126, 263 

Binary Compounds 15, 18 

: Bitumen 72, 169, 205 

I Blanchard, V 278 

I Blanchard’s Brush 253 

I Bleaching Powder 83, 289 

I Blue Process 266, 402 

I Blue-Vitriol 86 

Bolton, W. B . . 175 

Books on Chemistry 39 

j Borax 148 

I Borda, E 214 

' Borax Toning . . 343 

Bovey’s Toning Bath 365 

i Bothamley, C. H 186, 203 

! Bow, R.H 387 

I B)-adforde, Geo 347 

I Bra ham, P 203 

j Braun, A. et Cie 241, 261 

! Brenzcatechin 135 

I Brebner, H 203 

Bromides as Restrainers 218 

I Bromine 191 

! Brins, M 36 

Brooks, W 296 

Brush Toning 369 

Bunsen Burner 24, 29 

Burgess, J 176, 219 


422 


INDEX. 


PAGE 

Burton^ W. K 143 

Burton, C. I .... 323, 325 

Burnett, C. y. ...... . 256, 260, 344 

Burnishing Prints 255 

Burbank, Rev. W. H 355 

Burton’s Reducer 294 

Calomel 118 

Calotype 170, 171, 209, 306 

Caranza, M 339 

Carbon Printing 258 

Carbutt,J 246 

Carte-de-visite 255 

Castner, H, G 147 

Catechol 135 

Caustic Potash 130 

Caustic Soda 150 

Causes of Fading .... 405 

Cavendish, H 37 

Celluloid 179 

Chemicals .44, 45 

Chemical Elements 9, 10, 12 

Chemical Force 8 

Cherrill’s Toning Bath 366 

Cinnabar 117 

Chlorine 183 

Clark, Lyonel 248, 278 

Chinolin Blue 86 

Chloride of Lime 350 

Chlorus 12 

Chondrin 94 

Chrome Alum 84 

Collodio-Chloride of silver . . . 256 

Collodion Process 172, 211 

Collodion Emulsion 175 

Combination 18, 19 

Compounds 20 

Conductors 12 

Constant Proportions 18 

Condy’s Fluid Reducer 290 

Condy’s Fluid 133 

Constable, J. C 386 

Copper Chloride Reducer 286 

Copper Sulphate Reducer 287 

Copperas Ill 

Copal 158 

Corrosive Sublimate 117, 311 

Courtois, 107 

Corpuscular Theory 228 

Crookes, W 174, 191, 235 

Cryolite 66 

Cyanide for Fixing 399 

Cyanotype 266, 402 

Daguerre, LJ. M. 206, 207, 209, 37 7, 392 

Daguerreotype 170, 199, 204 

Daguerreotypes Toned 334 

Davanne, 356, 358 


PAGE 

Davy, SirH. 73, 126, 147,168,247,376 


Debenham, W E . . . 287, 296 

Decantation 26 

Development... 204, 213, 214, 224 

Disde'ri, AJ 313 

Distillation 26, 62 

Distilled Water 162 

Divers, Dr 105 

Divisibility 3 

Dixon Cf Gray 241 

Donuy, F 314 

Draper, Dr. . . ■ 196 

Dresser, A. R 325 

Drinkwater, T. W 200 

Dry Collodion 173 

Dry plates (Gelatine) 219 

Duhauron, Dticos 240 

Dunmore’s Toning Bath 368 

Durand’s Toning Bath 365 

Eastman Co 180 

Eau de Javelle 150, 412 

Eau de Javelle Reducer 289 

Eau de Labarraque 150 

Eder, Dr 264, 328 

Eder’s Reducer 293 

Edwards, B. J. Ca Co 241, 246 

Edwards’ Intensifier 325 

Egg-albumen 60 

Egli, 105 

Electrical Theory 200 

Elements 9, 12 

Elimination of hypo 409 

Elliot, Dr. A. H 74 

Emulsion 175, 176 

Emulsion Dry plates 219 

England, W 298 

Eosin 238, 240 

Eoside of silver 243 

Epsom Salts 116 

Ether 228 

Evaporation 23 

Evrard, Blanquart 248, 255 

Extension 4 

Fabficius 188 

Fading of Photographs 404 

Faraday, Prof. ... 334, 378 

Fargier, 260 

Farmer, H 291 

Ferguson’s Toning Bath 367 

Fermentation 61 

Ferric Chloride Reducer 284 

Ferridcyanide Reducer 290 

Ferric Oxalate Reducer 292 

Ferric Sulphate Reducer 285 

Ferro-gelatine 213 

Ferrous Oxalate 222 


INDEX. 


423 


PAGE I 


Ferrous Sulphate 212 

Filtration 24 

Fixing Processes 375 

Fixing with Ammonia 383 

Fixing with Common Salt. .377, 382 

Fixing with Cyanide 399 

Fixing with Hydrochloric Acid. 402 
Fixing with Hyposulphite of 

Soda 389 

Fixing with Potassium Bromide 383 
Fixing with Potassium Iodide. 382 

Fixing with Sea-salt 378 

Fixing with Water 402 


Fizeaii, h. L 335 j 


Flandreau’s Eliminator 411 

Flash-light 35 

Fluor-spar 90, 102 

Fog 210, 226 

Forces of Nature 7 

Formula 14 

Fractional Distillation 27 | 

Frankla?id, E 203 | 

Fraunhofer, 230 ! 

Fulminating Silver 145 

Fuming 214, 215, 248, 349 

Fusel Oil 64 


Fyfe, Dr. A 383 


Gallic Acid 209 ; 

Gallo-nitrate of Silver 171, 209 i 

Gaudin, M 175, 400 | 

Gelatine 179, 199 I 

Gelatino-chloride of Silver 257 

Gelatine Emulsion 176 , 

Gelatine Dry-plates 176, 219 

Gelis’ Salt 338 * 

Gifford. H.J 202 ! 

Girard, A 356, 358 | 

Girod, M 174 

Glacial Acetic Acid 59 

Glass 28, 29, 30, 31 

Glass-blowing 28 

Glauber, 188 

Glover, J 214, 215 

Glucose 154 

Glue 94 

Glycocoll 213 

Goddard, J. F 170 

Gold Residues 48 

Gotz.J.R 236, 246 

Graphite 80 

'Gray, Gustave Le 248, 350 

Green, A. G 134 

Green Vitriol Ill 

Grove, Sir IV. R 228 

Gun-cotton 137 

Guntz, M 142 ! 

Guthrie, F 184, 203 ; 


PAGE 

Halleur, Dr. 313 

Haloids 181 

Hanna ford, 344 

Hanson, IV 290 

Hartshorn 69 

Hardwich, T. F. .174, 196, 198, 

253, 283, 337, 342, 413 

Hart, F. W 410 

Heat 6 

Heisch’s Toning Bath 367 

Heliography 73, 169, 206 

Herschel, Sir J 266, 310, 389 

Hey wood. John 369 

Hodgkinson, IV. R 186 

Holmes’ Ozone Bleach. 121, 287, 413 


Hughes, A 352 

Huebl, Gaft 318 

Hunt, R 112, 185, 212, 311 

Hydrates 16 

Hydrogen 3, 36 

Hydrometer 62 

Hydroxides 16 

Hypochlorite Reducers 288 

Hypochlorites as Eliminators.. 410 

Hypo Eliminators 163, 409 

Hypo (How to Detect) 414 

Hyposulphites 391 

Hyposulphite of soda 387 


Iceland Spar 77 

India Rubber 79 

Inorganic 9 

Intensification 305 

Intensifying Gelatine Negatives 319 
Intensifying Collodion Nega- 
tives 307 

Intensifying Processes 305 

Iodine 4, 206 

Iodine as an Eliminator 412 

Isochromatic Photography .241, 244 
Isinglass 94 


Javelle Water 150 

Johnso7i, J. R 261 

Jones, Chapjnan 108 

Just, Dr 109 


Kallitype 266, 269 

Ke7i7iett, R 176, 219, 284 

Khig, H. N 174 

Knop, 179 


Laborde, Abbd 260, 345 

Lac 138 

Lainer’s Reducer ... 294 

Lainer, A 403 

Lakes 194, 202 


424 


INDEX. 


PAGE 

Lambert, Rev. F. C 240 

Latent Image. . . .181, 185, 188, 

196, 198, 201, 202, 224 

Laurie, A. P 320, 325 

Lavoisier 34 

Lea, Carey 124, 

139, 170, 191, 194, 198, 

200,203, 206, 213, 222, 318, 353 

Lead Intensifier 328 

Leaper, C 195, 203 

Leahy, T. M 214, 215 

Lewis’ Toning Bath 367 

Liebig’s Condenser 27 

Liebig 135 

Light 6, 181, 182 

Liesegang, Dr. R. E 330 

Liesegang, R. E 265 

Light 228, 229 

Lime Toning Baths 350 

Litmus 16 

Local Intensification 331 

Local Reduction 295 

Lloyd, C. B 412 

Llewelyn, J . D 174 

Lunar Caustic 144 

Luna Cornua 182 

Ltessac, Gay 86 

Lyte, Maxzvell 174, 314, 348 

Maconochie, Prof 313 

Maddox, Dr 124,176, 219 

Magnesium 35 

Magnus, Albertus 188 

Malhnann, Dr 325 

Mansfield, G 176, 219 

Mat he son, H 399 

Matter 3, 4, 5 

Matt-surface Paper 247 

Maynard 85 

“Measles” in Printing 349 

Meldola, Prof 87, 

207, 211, 324, 300 

Mercury 207 

Mercury Bichloride 310, 320 

Meta-bisulphite of Potash 131 

Meta-gelatine 94 

Metals 12, 15 

Methylated Ether ^8 

Methylated Spirit 64 

Minchin, Prof. 200, 201 

Mixtures 19 

A/oissan, M. II 90 

Molecules 5, 6. 19 

Molecular Motion ... . 6, 7 

Molecular Weights 14 

Alonckhoven, Dr. Von 198 

Morochini 190 

Moser, M 196, 198 


PAGE 

Multiple Proportions 18 

Muriatic Acid 102 

Nascent Silver 210 

Nascent State 224 

Neck, L. Van 226, 2^:7 

Newton, Sir IV 413 

Nicol, Dr. W. W. f 269 

Niepce, J. A^. .73, 169, 204, 206, 376 

Niepce, de St. Victor 61, 171 

Niepceotype 206 

Nitre ]3i 

Nitrogen 37 

Nomenclature 14 

Non-metals 12 

Obernetter, J. B 257, 331 

Oil of Lavender 205 

Oil of Spike 120 

Oil of Vitriol 155 

Optica] Sensitizers 243, 245 

Organic *j 

Organifier. 101 

Orthochromatic Photography. . 228 

Ossein 93 

Over-printed Proofs (Reduc- 
tion) 297 

Oxalate Residues 53 

Oxidation 35 

Oxygen 33, 36 

Oxymel 174 

Oxysalts 185 

Ozone Bleach. 287, 413 

Paracelsus 37 

Parkes 179 

Permanent Gases 38 

Permanganate of Potash.. .132, 

329, 414 

Peroxide of Hydrogen 105, 409 

Phenol 80 

Photogene . 175 

Photogenic Drawing 209, 247 

Photo-salts 191, 208 

Physical Development 213 

Physical Forces 7 

Pitteurs, M. de 178 

Pizzighelli Process 277 

Platinotype 272, 402 

Platinum Residues 52 

Platinum Toning 278, 339 

Poitevin, A 259, 264 

Pollock, 248 

Ponton, M 258, 263 

Porosity 4 

“ Potash” 128 

Potassium Bromide 222 

Bouncy, John,... 259 


INDEX. 


4:25 


PAGE I 

Powder Process 330 

Precipitation 24 

Preservative 101, 174 

Pressure 201 

Priestley t Dr 33 

Printing in Silver 247 

Protosulphate of Iron 212 

Pyroligneous Acid 59 

Quicklime 78 

Quicksilver 116 

Quinolin 86 

Quinol Intensifier 318 

Reactions of Developers 227 

Reade, Rev. J. B 92, 171, 209 

Realgar 196 

Reissigy Dr 415 

Red Prussiate of Potash 129 

Red Precipitate 7, 15, 33, 118 

Reducing Agent 209 

Reducing Processes 282 

Reduction 282 

Reduction of Residues 49 

Reduction of Prints 297 

Report of Committee on Fad- 
ing 404 

Residues ...47, 52, 54 

Resins 158 

Restrainers 210, 212 

Robinson, R, W 289 

Rochelle Salt 271 

Rodinal 122 

Roll-holder 179 

Roscoe, Sir H 157 

Ross, A 171 

Roy, Paul. ... 142 

Ruby Glass 96 

Russell, Major... 115, 214, 215, 217 

Sal-ammoniac 183 

“Sal-soda” 149 

Salt 20 

Salts 16, 17, 22 

Saltpetre 131 

Salt of Sorrel 132 

Sal-volatile 70 

Sand-blast 208 

Sarony’s Toning Bath 364 

"Saturated 23 

Sayee, B. J 175 

Scheele . . .82, 102, 182, 188, 209, 248 

Schlippe’s Salt 152, 318, 326 

Schonbein 121 

Schulze, J. H 247 

Scott, W. L 255 

Sea-salt for Fixing 377 

Seeley, E 352 


PAGE 

Sel-d’or 100, 338, 357 

Selle, H 328 

Sensitizers 172, 174, 176, 

179, 190, 206, 210 

Sensitizing Paper 248 

Shadbolt 174 

Silicate of Soda 152 

Silver Bromide 178 

Silver Haloids 181 

Silver Printing ... 247 

Silver Residues 49, 52 

Simpson, G. IV 256, 330 

Smith, Dr. A 409 

Snelling, H 401 

Sodium Hypophosphite 193 

Sodium Sulphite for Intensify- 
ing 322 

Soluble Glass 152 

Solution , 22 

Sound 8 

Spectroscope... 230 

Spectrum 232 

Spiller, A 105 

Spiller, G 351 

spiller,] 174, 397, 398 

Spiller’s Reducer 286 

Spring, Prof 201 

Stains Removed 297 

Starch-iodide Test 415 

Starnes, H. S ... 199 

Stas 178 

Stassfurt 128 

States of Matter. 5 

Stillman, . 284 

Sub-bromide of Silver 224 

Sub-oxide of Silver 189 

Sub-salt Theory 188, 189 

Sucrose 154 

Sugar of Lead 112 

Sulphites 156 

Sulphite of Soda (Acid) 403 

Sulphuretted Hydrogen 104 

Sulphuric Ether 88 

Sulphur Toning 357 

Sutton, T 259, 351 354 

Swan,J. IV 260 

Symbols 11, 12 

Tailfer Attout 88, 240, 246 

Talbot, Fox.... 61, 92, 168, 170, 

171, 207, 209, 247, 248, 

258, 379, 394 

Talbotypes 306 

Tannin 175 

Taupenot,J.M 174 

Taylor, Dr 253 

Tessie du Motay 35 

Tests for Hypo 414 


426 


INDEX. 


PAGE 

Thallium 191 

Thiosulphates 151 

Thiosulphuric Acid 150 

Thompson^ F. F 214 

Thomson, Sir W 6 

Thorpe, Prof 136 

Titte^'ton, 313 

Toning with a Brush 369 

Toning Processes 334 

Toning Baths 342-354 

Toth, V 328 

Turmeric Paper 68 

Turnbull’s Blue 269 

Tungstate Toning Bath. ....... 352 

Uranium 328 

Valentine, G. W 52, 331 

Vibration of Atoms 198 

Vidal, L 246 

Vinegar 59 


PAGE 

Vitriol, Oil of 155 

Vogel, H. W. . 108 189, 237, 242 

295, 412 

Volatile Alkali 68 

Waterhouse, Col 237, 342 

Water of Crystallization 17 

Weak Prints 362 

Wedgwood, 1 247, 376 

Wellington, J. B. B 327 

Wet Collodion 172 

Whitening 77 

Wi^gin, J. C 203 

Willis, W 222, 272, 293 

Williams, W. C 290 

Witt, Dr 87 

Wohler 189 

Worthy, Col. S 256 

Wothlytype 256 


Yellow Prussiate of Potash . . . 130 





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4 



“ It is interesting as a novel and of vastly more value.” — Rev. W. H. Burbank. 

” It is a book well worth reading, and should be in the hands of every live photog- 
rapher.”—}, R. Swain, 

” Every lover of photography will possess it.” — The Philadelphia Photog-rapher. 

” The book is an interesting contribution to the growing list of photographic litera- 
ture.” — The Brooklyn Times. 

” The book is well written, well printed, prettily bound, and what is better, contains a 
complete, true and instructive account of the discoveries and successive improvements of 
all the processes employed since the beginning of our beautiful art.” — P. C. Duchochois. 


“A HISTORY OF PHOTOGRAPHY,” 

WRITTEN AS A PRACTICAL GUIDE AND AN INTRODUCTION 
TO ITS LATEST DEVELOPMENTS. 

{Number Twenty-three of The Scovill Photographic Series.) 

By W. JEROME HARRISON, F.G.S., 

And containing a full-page portrait of the Author, with a 
Biographical Sketch by W. I. Lincoln Adams. 


COISTTEISITS. 


Introduction. | 

Chapter I. — The Origin of Photog- ! 

raphy. i 

Chapter II. — Some Pioneers of i 
Photography — Wedgwood and 
Niepce. 

Chapter HI. — The Daguerreotype 
Process. 

Chapter IV. — Fox-Talbot and the 
Calotype Process. i 

Chapter V. — Scott-Archer and the ! 

Collodion Process. 

Chapter VI. — Collodion Dry- 
Plates, with the Bath. i 

Chapter VII. — Collodion Emul- 
sion. I 

Chapter VIII. — Gelatine Emulsion 
with Bromide of Silver. 

Chapter IX. — Introduction of Gel- j 
atino-Bromide Emulsion as an i 


Article of Commerce by Burgess 
and by Kennett. 

Chapter X. — Gelatine Displaces 
Collodion. 

Chapter XI. — History of Photo- 
graphic Printing Processes. / 

Chapter XII. — History of Photo- 
graphic Printing Processes (Con- 
tinued). 

Chapter XHI. — History of Roller- 
Slides and of Negative-Making 
on Paper and on Films, 

Chapter XIV. — History of Photog- 
raphy in Colors. 

Chapter XV. — History of the Intro- 
duction of Developers — Sum- 
ming up. 

Appendix. — Dr. Maddox on the 
Discovery of the Gelatino-Bro- 
mide Process. 


The book is uniform in size of type and page with the other 
numbers of Scovill’s well-known Photographic Series. Bound 
substantially in cloth, with gilt imprint. 

PRICE, - _ = $1.00. 


“ It has the rare merit of being both concise and comprehensive.” — W. H. Sherman. 

” The work is a most valuable and interesting addition to our photographic literature.” 
— The Photographic Eye. 

” Any one who would like to read the history of one’s profession — and who would not 
—will find much to enjoy in this book, and much of profit as well.” — The St. Louis 
Photographer. 

” It presents in a brief and comprehensive way the origin and development of this art, 
with its consequent theories and experiments, and will be ol value and interest.”— 7"^^ 
Independent. 

” The story is told in an interesting style, and with such copious references that those 
who have the time and inclination can readily enter into more deeply upon the subject, 
and follow the course recommended.” — The Philadelphia Public Ledger. 


1 


PHOTOGRAPHIC PHBLIGATIONS. 

For Sale by The Scovill & Adams Company. 


Price 
per copy. 

LANTERN-SLIDES, AND HOW TO MAKE THEM.— By A. R. Dresser. A 

new book, very complete and practical $0 25 

FLASH-LIGHTS, AND HOW TO MAKE THEM. By L. C. Bennett. A 

thoroughly practical book, fully illustrated 50 

BROMIDE PAPER AND HOW TO USE IT. A practical treatise, written by 

an expert, with a full-page illustration. Price, postpaid 25 

THE KNACK. — Written to help the beginner out of difficulty Reduced to 25 

PHOTOGRAPHIC LENSES; THEIR CHOICE AND USE.-J. H.Dallmeyer. 

A special edition edited for American photographers. In paper covers 25 

THE CHEMISTRY OF PHOTOGRAPHY.— By Prof. Raphael Meldola 2 00 

THE LIGHTING IN PHOTOGRAPHIC STUDIOS.-By P. C. Duchochois. . . . 75 

THE PHOTOGRAPHIC IMAGE.— By P. C. Duchochois 1 50 

Cloth bound 2 00 

THE FERROTYPER’S GUIDE. — For the Ferrotyper, this is the only standard 

work. Seventh thousand 75 

THE PHOTOGRAPHIC STUDIOS OF EUROPE.-By H. Baden Pritchard, 

F.C.S. Paper cover 50 

Library Edition 1 00 

ART OF MAKING PORTRAITS IN CRAYON ON SOLAR ENLARGE- 
MENTS. — (Third Edition.) By E. Long 1 00 

PHOTOGRAPHY APPLIED TO SURVEYING.— Illustrated. By Lieut. Henry 

A. Reed, U. S. A. Cloth bound 2 50 

HISTORY AND HAND BOOK OF PHOTOGRAPHY.-Translated from the 

French of Gaston Tissandier, with seventy illustrations. Cloth bound 75 

CRAYON PORTRAITURE.— Complete instructions for making Crayon Portraits 
on Crayon Paper and on Platinum, Silver and Bromide Enlargements ; also 
directions for the use of Transparent Liquid Water Colors, and for making 

French Crystals. By J. A. Barhydt. A new edition. Paper covers 50 

Cloth bound 1 00 

ART RECREATIONS. — A guide to decorative art. Ladies’ popular guide in 

home decorative work. Edited by Marion Kemble 2 00 

AMERICAN CARBON MANUAL.— For those who want to try the Carbon printing 

process, this work gives the most detailed information. Clothbound. Reduced to 50 

MANUAL DE FOTOGRAFIA. — By Augustus Le Plongeon. (Hand-Book for 

Spanish Photographers.) 1 00 

SECRETS OF THE DARK CHAMBER.-By D. D. T. Davie 50 

THE PHOTOGRAPHER’S BOOK OF PRACTICAL FORMULAS.— Compiled 

by Dr. W. D. Holmes, Ph.B., and E. P. Griswold. Paper covers 75 

Clothbound 1 50 

AMERICAN HAND-BOOK OF THE DAGUERREOTYPE. — By S. D. 
Humphrey. (Fifth Edition.) This book contains the various processes 
employed in taking Heliographic impressions Reduced to 25 

THE PRACTICAL PHOTOGRAPHIC ALMANAC FOR 1879 25 

MOSAICS FOR 1870, 1871, 1872, 1873, 1875, 1885, 1886, 1887. 1888, 1889 25 

BRITISH JOURNAL ALMANAC FOR 1878, 1882, 1883, 1887, 1891 25 

PHOTO NEWS YEAR BOOK OF PHOTOGRAPHY FOR 1871, 1876, 1887,1888, 

1890, 1891 25 

THE PHOTOGRAPHER’S FRIEND ALMANAC FOR 1873 25 

ii 


Wilson’s Photographic Pnbllcatlons. 


For Sale by Tbe Scovill & Adams Company. 


WILSON’S PHOTOGRAPHIC MAGAZINE.— A semi-monthly magazine 
devoted to the advancement of Photography. Edited for twenty-eight 
years by Edward L. Wilson, Ph.D. Gives almost 800 pages of 
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cess cuts, all of great interest to every camera worker, during the 
year. Issued first and third Saturdays of each month. ^ Price, $5.00 
per year ; $2.50 per half year. Subscriptions may begin any time- 

WILSON’S QUARTER CENTURY IN PHOTOGRAPH Y.— A com- 
plete text-book of the art. Twenty-four hand-books in one volume, 
upon ever}" branch of Photography ; 528 pages, profusely illustrated, 
with notes and index. Price, post-paid, $4.00. 

WILSON’S PHOTOGRAPHICS. — “Chautauqua Edition,” with Appen- 
dix. By Edward L. Wilson, Ph. D. Eighth Thousand. Covers ever)’ 
department. Altogether different from “Quarter Century.” Fully 
illustrated, with notes and index. Price, post-paid, $4.00. 

PHOTO-ENGRAVING, PHOTO-ETCHING, AND PHOTO-LITHO- 
GRAPHY. — By W. T. Wilkinson. Revised and enlarged by Ed- 
ward L. Wilson, Ph.D. The most practical work extant on these 
subjects. (Send for detailed contents list.) Price, post-paid, $3.00. 

ESSAYS ON ART.— Composition, Light and Shade, and the Educa- 
tion OF THE Eye. — By John Burnet. Three priceless volumes in 
one, with lob illustrations, lithographed in facsimile from original 
costly edition. $4.00, post-paid. 

THE BOOK OF THE LANTERN.— By T. C. Hepworth. The most 
practical handbook to lantern work so far issued. 278 pages. Bound 
in cloth. Price, $2.00, post-paid. 

PHOTOGRAPHIC MOSAICS. — An annual record of Photographic pro- 
gress. Edited by Edward L. Wilson, Ph D. Issued every Novem- 
ber ; now in its twenty-eighth year. Universally acknowledged to be 
a most helpful annual. Price, paper, 50c.; cloth bound, $1.00, * 

iii 


1890 . ( 889 . 1888 . ( 887 . 


THE PHOTOGRAPHIC TIMES ANHHALS 

ARE 

A Record of Photographic Progress. 


Price, per copy, _______ 

I.,itorary Kdition, _______ 

Hdition die I^uxe, _______ 

By mail, 12 cents extra. 


50 

1 00 

2 50 


Contains five full-page illustrations — 

All Hxquisite Plioto-Grayure, by Ernest Edwards. 

A Bromide Print, by the Eastman Company. 

A Silver Print, by Gustav Cramer, of St. Louis. 

TTwo Mosstypes, by the Moss Engraving Company. 
197 pages of Contributed Matter consisting of articles on various subjects, by 80 repre- 
sentative photographic writers of this country and Europe. 


Contains eight (8) full-page high-grade illustrations ; and over ninety (90) original con- 
tributions, written expressly for its pages, by the most eminent 
s photographic writers of Europe and America. 

THE ILLUSTRATIONS COMPRISE: 

A Plioto>lL,ltliosrx*npli, showing an improved new process, by the Photo- 
Gravure Company of New York. 

A Plioto-Copper-Plate of a Pictorial Landscape Subject, 

by E. Obernetter, of Munich. 

A MeisetiPacli of “The Old Stone Bridge,” by Kurtz. 

A S^inc Htdiinsf, from the Engraving, which is itself as fine as an Engraving, 
by Stevens & Morris. 

A Cliarmiiisr Ctiild Portrait, by Crosscup & West’s improved process. 
TTliree Mosstypes of popular subjects. And 

330 PAGES OF VALUABLE INFORMATION. 


ENTIRE EDITION SOLD. 


/ Contains the Following Full-Page Pictorial Plates 

“ 'Tliomas Hdisoii.’ A Portrait of the Eminent Electrician. George M. Allen 
& Co., New York. 

“ BaPybood.’* A Tinted Photo-Gravure. The Photo-Gravure Co. of New York. 
“ Putnam’s Escape.” A Collection of Historic Views. The Crosscup & West 
Engraving Company, Philadelphia. 

“ Southern Emit.’ An Orthochromatic Study. The Electro-Light Engraving 
Company, New York. 

” At the Barracks.” A copy of the great Meissonier picture. William Kurtz, N.Y. 
” Minstrel Party at ’John Brown’s Eort.’ ” Photo-Engraving Com- 
pany, New York. 

“John Brown’s Home and Grave.” Lewis Engraving Co., Boston. 

' OIF Buty.” An Instantaneous Study. William Kurtz, New York. 

” Minnehaha Ealls in "Winter.” Levytype Company, Philadelphia. 

” Central Park.” In the Menagerie. I. M Van Ness, New York. 

” A Merry Xale.” A Child Group. F. Gutekunst, Philadelphia. 

” Xhe "Van Rensselaer Manor House.” Photo-Electro Engraving 
Company, New York. 

” An Improvised Studio.” Electro-Tint Engraving Company, Philadelphia. 
” Xhe Bats.” A “ Flash ” Light Photograph in Howe’s Cave. William Kurtz, N.Y. 
” A Raider’s Resort.” Morgan’s Favorite Rendezvous. M.Wolf, Dayton, Ohio. 
” Group of Esquimaux.” William Kurtz, New York. 

“Diatoms.” Photo-Micrographs. William Kurtz, New York. 

“ Xropical Euxuriance.” A Scene in Florida. Moss Engraving Co., N. Y. 
“ An Arctic Camp.” Moss Engraving Company, New York. 

^ “ Home of Edsrar Allan Poe.” Moss Engraving Company, New York. 

NEARLY 400 PAGES OF READING MATTER. 


d 

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10 w 

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0. 


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IV 


THE 


American Annual of Photography 

and Photographic Times Almanac 

FOR 1891. — 

LARGER AND BETTER THAN EVER BEFORE. 

OTer TMrty-six FOLL-PASE lUusirations. over Oae Hundred Original Contridniiens. 

price: xhe: saimle: as csuae,. 

In Paper Covers, 50 cents. Library Edition (cloth bound), $1.00. 

By Mail, 15 cents extra. 


SOME OF THE PICTORIAL ILITTSTRATIONS : 


A Fine Copper-Plate Engraving (Portrait Study). By the New York Photo-Gravure 
Company. 

“Attraction,” “Temptation,” “ Satisfaction,” a series of three hunting pictures. By 
R. Eickemeyer, Jr. 

The Solar Eclipse (December 22, 1889I. By Prof. S. W. Burnham. 

“Three Little Kittens.” By William M. Browne. 

“ The County Fair.” By J. P. Davis. 

A Portrait of Prof. Burnham. By Hill & Watkins. 

“ I Love ’ 00 ,” (a charming child picture). By Franklin Harper. 

Daguerre Portraits. (Nine portraits of J. L. M. Daguerre, including one never before 
published) 

The Yacht “Volunteer,” Before the Wind. By H. G. Peabody. 

Finish of Race Between Taragon and St. Luke. By J. C. Hemment. 

“ Enoch Arden.” A Portrait Study. By H. McMichael. 

“ The Life Class.” By Charles N. Parker. 

Portrait Study. By William Kurtz. 

“ The Regatta.” Two Yachting Pictures. By A. Peebles Smith. 

A “ Flash ” Picture. (Interior.) By Horace P. Chandler. 

“ Contentment.” By Miss Emilie V. Clarkson. 

Old Mill on the Bronx River. By John Gardiner. 

“ Sailing the High Seas Over.” By Harry Platt. 

The Great Selkirk Glacier Face. By Alexander Henderson. 

“Lightning.” (Two Pictures.) By W. N. Jennings. 

“ Down in the Meadows.” 

“ Forest Shadows.” By G. De Witt. 

“In Chautauqua Woods.” By “A Chautauquan.” 

Haines Falls. By W. S. Waterbury. 

Besides many Pictures throughout the Advertising pages. 


IS 

IT 

NOT 

SOP 


That Americans like the best of everything, and when the best costs the least 
they will buy it without urging. 

The more distinctively American such an article is, the greater will be their 
pride in it. 

It goes without saying that a full- jeweled watch is worthy of a good case, and 
that an Encyclopedia should be bound in something more durable than 
paper covers. 

The American Annual of Rhotog^rapliy is nowin world-wide 
favor, and commonly spoken of as an ” Encyclopedia of Photographic 
Progress.” 

It should be ordered with cloth binding (Library Edition), as it has, both in 
bulk and importance, outgrown paper covers. Other books, containing no 
more pages or information, sell for $3.00. In attractiveness they will not 
compare with 


The Photographic Times Annual for 1891, which is the most profusely 
and handsomely illustrated Photographic Book ever published. 


V 


THE 

American Annual of Photography 

and Photographic Times Almanac. 

“THE GREATEST ANNUAL ON EARTH” 

(AS IT HAS BEEN CALLED) 

FOR 1893 IS GREATER THAN EVERT 

It contains over two dozen full-page pictures by the best represent- 
ative photographic reproduction printing processes, and 120 original 
articles on practical subjects, by the best photographic writers and workers 
of the world. 227 pages of instructive and interesting reading matter. 


NEW TABLES! NEW FORMULAS! NEW METHODS! 

The Standard Formulas and Useful Receipts have been greatly aug- 
mented, entirely re-arranged and thoroughly revised, and the entire book 
for 1892 goes out to the reader much more conveniently arranged, better 
printed, and containing more valuable and interesting matter than ever 
before. 


A LIST OF THE ILLUSTRATIONS: 

Flirtation,” by H. McMichael ; New York Photogravure Co. 
“Don’t be Afraid!” by Gustav Leupelt ; F. Gutekunst. 

‘‘A Portrait Study,” by PYiedrich Muller; Albertype Co. 

“ Uncle Ned,” by R. Eickemeyer, Jr. ; Geo. M. Allen & Co. 

“At Play,” by Lieut. Karl Hiller ; Wm. Kurtz. 

“Herr Nesper as ‘Wallenstein,’” by Heinrich Riffarth. 

“Grace Ideal,” by Harry L. Ide ; Electro-Light Engraving Co. 
“Bye-Bye, Papa!” by James E. Line; Electro-Tint Engraving Co. 
“What a Waterbury Lens Can Do,” by Andrew B. Dobbs ; N. Y. 
Engraving and Printing Co. 

“Village Scene in Austria,” by the Interior Court and State Printery 
of Vienna. 

‘•Engaged?” by the Crosscup & West Engraving Co. 

“ Swiss Village Street,” by Ellerslie Wallace ; Moss Engraving Co. 
“Mechlin Cathedral, Belgium,” by Ellerslie Wallace; Moss En- 
graving Co. 

“On the Via Mala, Switzerland,” by Ellerslie Wallace ; Moss En- 
graving Co. 

“A Torpedo Well,” by Erastus T. Roberts ; Wm. Kurtz, New York. 
“Roasting Apples,” by Louis C. Bennett; Photo-Electro Engraving Co. 

“An Athletic Photographer,” (S. J. Dixon) J. C. Hemment ; W. 
Kurtz. 

“ A Baden Highland Peasant,” Oscar Suck ; Electro-Light En- 
graving Co. 

“ A Stage Beauty,” by Stholl ; Photo-Engraving Co. 

“ Blankenberghe Beach,” by Alfred Canfyn ; M. Wolfe. 

“An Old Roman Garden,” W. J. Stillman ; Crosscup & West En- 
graving Co. 

“ The Little Maid from School,” by F. Gutekunst; The Levytype Co. 
“ Doubles,” by A. A. Adee ; The Levytype Co. 

“ A Moorish Girl,” by The Levytype Co. 


OUT OF PRINT. 


VI 


THE 


American Annual of Photography 
and Photographic Times Almanac 

FOR 1893. 

THE GREATEST AMOUNT OF PHOTOGRAPHIC INFORMATION. 

THE GREATEST NUMBER OF ILLUSTRATIONS. 

THE GREATEST NUMBER OF PHOTOGRAPHIC ADVERTISEMENTS. 
MAKING IT THE “ GREATEST ANNUAL ON EARTH.” 


IT CONTAINS 

THIRTY (30) PULL-PAGE PICTURES. 

OVER TWO HUNDRED (200) PAGES OP CONTRIBUTED 

ARTICLES, especially written for this volume by the best equipped 
photographers and photographic writers in two hemispheres. 

NEW TABLES, NEW FORMULAS, AND NEW METHODS. 
REVISED LISTS OF PHOTOGRAPHIC SOCIETIES. 
RECORD OF PATENTS, NEW BOOKS, and, in short, every- 
thing Relating to Photography. 

FILLING MORE THAN FIVE HUNDRED RAGES IN ALL. 

An indispensable Hand-book for the Photographer, young or 
old, Amateur or Professional. The First Edition is 18,000 
Copies ! This is an unprecedented demand for a photographic 
work, but the BOOK ITSELF is unprecedented in the ANNALS 
OF PHOTOGRAPHY ! ! ! 

The Price remains the same : 

Paper Covers, ______ $0.50 

Clotli-Bound (Library Hdition), - i.oo 

POSTAGE, 15 CENTS EXTRA. 

Putting it within the reach of all. 

vii 


lOO 


Odd Volumes 



OF 


THE 


PHOTOGRAPHIC 


TIMES 


Published between 1873 and 1884, 
each volume covering one year, 
bound in cloth with gilt stamp^ 
are offered for sale at ONE 
DOLLAR A VOLUME, express 
charges to be paid by purchaser. 
Address The Photographic Times 
Publishing Association, 423 
Broome Street, New York. 


ONE DOLLAR 


ENT to the Publishers by a new subscriber will 


obtain -THE PHOTOGRAPHIC TIMES” 


for Three Months. The regular subscription price 
is Five Dollars per annum ; single copy. Fifteen 
Cents. THE PHOTOGRAPHIC TIMES PUB- 
LISHING ASSOCIATION, 423 Broome Street, 
New York. 



Vlll 



Edited by W. I. LINCOLN ADAMS, 

IS THE 

ONLY ILLUSTRATED WEEKLY 

PHOTOGRAPHIC MAGAZINE 

IN THE WORLD. 

In the year fifty-two full page pictures 

are given, making THE PHOTOGRAPHIC 
TIMES the best illustrated Photographic 
periodical in the world. Special numbers contain 
more than one high-grade illustration ; and there 
are published, beside superb Photogravures, 
pictorial illustrations by the other photographic 
and photo-mechanical printing processes. 

The illustrations are carefully selected, and repre- 
sent the best work of representative American and 
foreign photographic artists. 

The Editorials are of greatest practical 
value as they are the result of actual practice and 
experiment by the staff, and the articles are by the 
most eminent authorities in this country and abroad. 

One YeaVf - - $5.00 | Months^ - - $2.50 

Three 3Ionths^ trial, - - $1,00 

THE PHOTOGRAPHIC TIMES PDBLISHIHG ASSOCIATION, 

FXJB3L.ISHIGR,S, 

4:23 Broome Street, New York City^ 


IX 



TWELVE PPUKHIPBIt STDDIES. 


SECOND EDITION. 


A COLLECTION OF PHOTOGRAVURES FROM THE BEST 
REPRESENTATIVE PHOTOGRAPHIC NEGATIVES 
BY LEADING PHOTOGRAPHIC ARTISTS. 

The Collection includes: 


“ Dawn and Sunset ” H. P. Robinson 

“ Childhood ” . . . H. McMichael 

“ As Age Steals On ” J- F. Ryder 

‘‘A Portrait Study” B. J. Falk 

“ Solid Comfort ” John E. Dumont 

“ Ophelia ” H. P. Robinson 

“No Barrier” F. A. Jackson 

“ El Capitan ” VV. H. Jackson 

“ Still Waters ” J . j. Montgomery 

“ Surf” James F. Cowee 

“ A Horse Race ” George Barker 

“ Hi, Mister, may we have some Apples ” Geo. B. Wood 


Printed on Japan Paper, mounted on boards. Size 
II X 14, in ornamental portfolio envelope. Price, $3.00. 
Sent post-paid on receipt of price. 

THE SCOVILL 4 ADAMS COMPANY, PuWiste. 

X 


THE 


(^l^aataaqaa S^liool of jpt^otograpl^y. 

motto: and there was light." 



LOUIS MILLER, President. JOHN H. VINCENT, Chancellor. 

MISS K. F. KIMBALL, Buffalo, N. Y., Secretary C. S. P. 

This School instructs in the Theory and Practice of the Art-Science of 
Photography, at the Chautauqua Assembly Grounds, in Summer ; the 
local classes at the School’s Headquarters, 423 Broome Street, New 
York City, during the Autumn, Winter and Spring, and by the corres- 
ponding classes through Printed Lessons and the Organ of the School, 

I. — The Correspond in gr Class, Headquarters 423 Broome Street, New York, open for 
admission at any time, receives instructions by twenty-four printed lessons, prescribed 
home practice, required reading and by correspondence with the instructor. 

Course of Instruction one year, tuition fee, inclusive of books, . . . . $7 00 

II. — The Practising Class opens on July ist, every year, and remains in session until 
about September 15th. Practice in studio and field. Theoretical instruction and lectures 


on photographic subjects. 

Course of ten lessons, $5 00 

Special lessons, each, i 00 


Independent of photographic materials and the text book. 

III. — The New York Classes begin November 15th and end about May 15th. The 
skylight room and laboratory used by these classes are on the seventh floor of No. 423 
Broome Street, New York. (Take elevator.) Separate classes for ladies. 

Cost of course of ten lessons, including entrance fee, text-book and materials 


used in demonstration, . . $7 50 

Special single lessons, per hour, each, . . . . ' i 00 

Cost of ten lessons in Portraiture, or special subjects, 10 00 


IV. — The Post-Graduate Class course of instruction two years. Subjects ; Chemis- 
try, Photo-Chemical Processes, Optics, and Esthetics by required reading and correspon- 
dence with the Instructor. 

Tuition fee, including one year’s subscription to The Photographic Times but 

independent of text-books, . . $10 00 

After completing a regular course the student is admitted to examination and, if 
passed, is awarded a Chautauqua Diploma. 

The weekly Photographic Times, illustrated, is the official journal of the school. 
Students residing in foreign countries will be charged $i extra for postage. 

In accomplishments and numbers the Chautauqua School of Photography stands un- 
rivaled. Her fame has reached beyond our own shores, for among the students of the 
Corresponding Class are many residents of Canada, the West Indies, Mexico, South 
America, Europe, India, China, Japan and South Africa, 

The Chautauqua Exchange Club, an institution of the School, has proved to be a 
very useful and instructive adjunct to the regular instruction . 

For particulars, address 

Prof. CHAS. EHRMANN, 

INSTRUCTOR C. S. P., 

423 Broome Street, New York. 


XI 


THE 

^coVill \ Adani^ Gompani), 

423 Broome Street, New York City, 

SUCCESSORS TO THE 

PHOTOGRAPHIC DEPARTMENT 

— OF THE — 

Scovill Manufacturing Company. 

ire Mannfactarers, Importers of and Dealers in 

AN UNEQUALLED VARIETY OF 

PHOTOGRAPHIC GOODS, 

EMBRACING 

Every Requisite of the 

Practical Photographer, 

Professional and Amateur. 


PUBLICATION DEPARTMENT. 

Publishers of “THE SCOVILL PHOTOGRAPHIC SERIES” 
(44 publications), the “Photographic Times Annual,” etc., etc. 

Latest Catalogue of Photographic Books and Albums, and a 
copy of “ How TO Make Photographs” sent free on application. 

W. IRVING ADAMS, H. LITTLEJOHN, 

President Treasurer. Secretary. 


XU 


-I*. ' 

I 




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'f 





I 9 


m. 




GETTY CENTER LIBRARY CONS 

NH 510 H32 1892 BKS 

c. 1 Harrison, William Je 

The chemistry of photography / 



3 3125 00164 3564 j 











